Guide to Co-operative Play

Note: These methods were established initially to make characters able to join and progress in the Warrior of Sunlight covenant, so those of you following this covenant will benefit most from this guide. Other players interested in learning how best to summon or be summoned for some fun or to farm some souls/humanity may also benefit. You may also find the guide useful if you are looking to get summoned for duels via Red Sign Soapstone or Dragon Eye, but are probably better served if you locate a summoning guide focused on PvP rather than jolly co-op. After some initial pointers, there will be a break-down of each area by boss (since the point of co-op is to help the host take down the boss), stating what levels are found to be the best to be summoned for that particular area and why or why not some bosses are better for being summoned than others.

Universal Pointers:
  1. Never drop your sign right in front of the boss fog unless nothing else is working. As a general rule, place your sign as close to the first bonfire before the boss area as possible. The most likely possibility of getting summoned at the boss arena instead are by hosts that have survived through the level but lost summons along the way, and who want to summon replacements without backtracking all the way to the bonfire. With only a few exceptions, it is better to help the host through as much of the level as possible for a couple of reasons. The first is that hosts that would need help with bosses have a greater chance of dying on the way to the boss area and losing their human form, and thus their ability to summon you. Second is that if you leave it right in front of the boss area, then hosts are more likely to summon others who have placed their sign earlier in the level instead of you.
  2. Know the boss and the area well. Don’t attempt to help a host fight Chaos Witch Quelaag without any Purple Moss Clumps for the journey there, don’t go into the Moonlight Butterfly without magic or arrows, and please do not try to help with the Centipede Demon without an Orange Charred Ring. Remember, you only benefit from assisting if you survive to the end, so make sure you’re actually prepared for what you’re trying to help with.
  3. Be able to heal yourself. This typically means investing in enough attunement and faith to at least cast the miracle Heal. Other options include having a lot of backup Humanity (the consumable item), a Sanctus shield for stretches where you are just travelling through the level, or using the Ring of the Evil Eye. Yes, the host can heal you with their Estus Flasks, but your job isn’t to rely on the host, it’s to assist the host. Put as little burden on them as possible and make sure you can take care of yourself.
  4. Look the part! While not as important as the rest, in some of the more populous areas this may be the deciding factor of someone choosing you over another phantom (remember, hosts can see your silhouette as they stand over your sign). When going up against the Demon Firesage, who requires a lot of mobility, would you rather summon the phantom who is wearing full Havel’s armor with a Grant, or the one in Gold-Hemmed robes with Logan’s Catalyst? Sure the guy in Havel’s may have invested in enough Endurance/have the right rings to be able to run and roll just as well as the one in lightweight robes, but you have no way of knowing that yet. Tying in with point #2, just look like you’re prepared for the area, it may just be the deciding factor.
  5. While any build can be used in co-op with varying effectiveness, if you are serious about co-op you should consider making specific co-op dedicated builds. Just as many players will make "PvP builds" or "PvE builds," a "co-op build" can be much more effective than one designed for other purposes that you just happen to co-op with from time to time. You could be the best player or PvPer in the world, and it won't matter if a mob or invader simply avoids you and kills the host before you can stop them. In addition to the co-op oriented Princess Guard miracles, there are a number of other spells and types of equipment that can go a long way towards keeping a host alive even in the face of a skillful invader or dangerous computer enemies, and that should be considered for any serious co-op build. The key to many is that your melee and spell attacks will not harm your host, but will often defeat or repel enemies or even cancel backstab attempts. Area of effect spells such as the firestorm type pyromancies, gravelord sword dance, or Wrath of the Gods/Force can be very useful in warding off an invader or preventing the host from being backstabbed. Vow of Silence is extremely effective at keeping a host from being struck by powerful incoming spells such as soul spears, Wrath of the Gods, or great combustion. Weapons with area of effect or ranged attacks such as the Golem Axe or Moonlight Greatsword can similarly save hosts from untimely deaths, and those with wide horizontal sweep attacks such as ultra greatswords are extremely effective at stopping backstab attempts provided you remember to stick close enough to the host to guard their back. By utilizing these types of spells and equipment, and by planning out your co-op character in advance, you can greatly increase your successes in getting a host through to the boss fog gate, and thereby enhance your overall jolly co-op experience.


Dealing with other players:


As is the nature with online play, you will be dealing with other actual players. These are your biggest unknown factors when going for a co-op session. Here are some points to cover some of the main issues that will arise, and how you can better secure a successful run.


Dealing with hosts:

  • Protect the host at all costs, including your own life. You should consider yourself the host's bodyguard for the duration, and always stay at least within sight of the host, if not closer.
  • If you get a host that is truly unprotectable, know that there was nothing you could do to save them and accept it. Maybe they charged blindly into the room with the Wheel Skeletons in the painted world. Maybe they thought the best way to take on Havel the Rock was to try and tank him. Maybe they just plain went out and rolled right off of a cliff into a bottomless pit. The host will have learned from their mistake, as is the nature of this game, and they may even have more humanity to be able to summon you again, but this time with greater competence in the run.
  • If you see a host that is clearly lost, try and assert yourself as the current “leader” of your group. Learn to communicate through your actions where the dangerous areas are and where it’s safe by keeping your shield down and running through calm spots, but slowing down to a walk and raising your shield as you approach a dangerous area. People can interpret a lot from your actions alone, so know how to use basic movement to communicate what’s what about the level.
  • Use your gestures. When basic movement isn’t enough, be sure to remember your many, many gestures. Is your host running ahead of you in the wrong direction or passing up some important treasure? Start heading towards the proper direction/treasure and use beckon. Is there some object only the host can interact with that they are not seeing? Use your point gestures, or some other directional prompt such a half blocking with a shield a few times in the direction of the object. Many things can be communicated to let a host better understand the world as opposed to messaging out of game.
  • Assume the host is going for everything in the level. Try to lead them down the most efficient path toward the boss, but be prepared to have some hosts with other goals, such as seeking your help in bringing down an optional miniboss such as Havel or the Undead Dragon in the Undead Burg and Painted World, respectively, or just going for all the loot in an area. Also, if a boss has a tail, assume they are going for cutting it off. If you notice the host is just blasting away at the Gaping Dragon or Priscilla with Pyromancy while you're dodging around trying for its tail, then you can go ahead and start just going for the kill, but make sure you know that the host is not going for something special first.

Dealing with invaders:
  • When you are first summoned, always sweep area around you carefully for invaders. Pay attention to how your host is acting; it is fairly common to be summoned into a world in which an invader has already appeared, meaning you may miss the official invasion warning message or other indicators and end up walking blindly into a trap. A common warning sign is if the host is facing a particular direction and keeps raising and lowering their shield. It usually makes sense to assume there is an invader lurking about even if you're not sure, just to be safe.
  • If you are near the boss door when someone invades, the easiest thing to do is to get the host to go through it, as this will send the invader back to their own world.
  • Your primary objective is to protect the host. As much as possible, do not let the host be the one to engage the invader. Keep in mind: if the host dies, you all lose. If you die, the host still has a chance to go on.
  • You do not need to be honorable, although sometimes it has its benefits. Some phantoms may be honorable and will allow you to finish off enemies, heal, and bow before engaging, but you are not here to PvP. If you like, bow back at your own risk. If you face an experienced invader or one with motives besides simply killing everybody, taking the time to be polite can buy time for the host to make it to the boss gate, or even occasionally get an invader to turn friendly. If you kill the invader though, there's no need to spam rude gestures, so do bow at this time.
  • Hide your numbers. Have one phantom act as bait, while another acts as the trap. DO NOT use the host as bait. When the invader goes for the trap, go out from the pillar you were hiding behind and backstab the invader--repeatedly!
  • Generally, you want to surround the invader. If all 3 of you corner him, there's nowhere for him to go and nothing for him to do but to die.
  • If you are acting as a host, try and summon phantoms with a variety of combat styles. A half-decent invader will be able to roll around and dodge 3 melee attackers. This task becomes infinitely more complicated if some of you are attacking him from range with bows, crossbows, and spells, another is going for melee, and another is turtling. Hosts and friendly phantoms are immune to one another's attacks and spells, so don't be hesitant to use an AoE spell or take a swing even if the invader is close to an ally. It is even possible to cast or attack through one another, sometimes fatally surprising an invader who was focused only on the foe right in front of him.
  • Learn where invaders usually appear. In most areas, invaders will summon in certain specific, set locations in the area depending on where the host is. By repeatedly aiding hosts through particular areas, you should develop an important sense of where invaders will likely appear at any give time, and therefore which route they will usually take to track down the host once they invade. This will enable you to better avoid ambushes or set your own, and also discern whether it will be safe or not to get the host to run directly to the boss fog gate.

And remember that invaders are players too. Other people. They're not AI, and more importantly they're not doing it just to be jerks. They're just people playing a different, necessary facet of this game. When all is said and done, should you lose out and die while assisting a host, know that the only thing you lost was some time. You don’t lose souls or humanity, and all you need to do is use your White Soapstone again to start over. Other players may mess up things for you, but don’t get upset because you didn’t really lose anything. If you fight hard and things still don't work out, the host may well summon you right away for another try. Bottom line, have fun with it!

Dealing with summons and being a host:
  • You can (and should) summon members of the Warrior of Sunlight covenant, who tend to specialize in co-op and are identifiable by their golden summoning symbols. If none are available, do not discredit those with white summon signs. There are a few covenants that also focus on co-op that do not change your summon sign color, so a white phantom can still be just as dedicated as a gold phantom.
  • Per the above advice for players to "look the part," it often makes sense to summon players that appear to be properly attired for the area or at least in possession of generally good equipment. You may view the potential summon's present equipment by standing on or very close to their summoning sign and taking a look at the resulting phantom image.
  • Some extremely skilled or advanced players prefer to challenge themselves by using obscure or minimal equipment, so while it may be a bit more of a gamble, do not be afraid to summon folks with strange getups if you feel lucky or already have one summon that seems up to the task. It may also make sense to ignore the equipment consideration if you are just entering a new area and are worried about invaders. In that instance, activating the first summoning signs you see just to get any kind of reinforcements can help.
  • If you only need help for a certain stretch of a level, try to convey that to any summons prior to or after dealing with that particular hurdle or miniboss. Abruptly using the black separation crystal without warning to send summons home can be perceived as very rude, particularly when the summon was anticipating a good boss fight (with corresponding rewards) or PvP with invaders. As a host, you are expected to know what equipment they were using (and therefore their likely fighting styles) when you made the choice to summon them, so not "liking" the summon is almost never an excuse. Having summons assist you will not reduce the number of souls you receive, so there is no reason to dismiss summons prior to defeating the boss.
  • Summons are players willing to volunteer their time and energy to help you out with a difficult game, and should be appreciated accordingly. Karma in Dark Souls can also be swift and terrible (and sometimes even takes the form of an invasion by an angry former summon), so if you think something went amiss it is often wise to send a quick thanks or apology message to the summon involved. On the PSN, for example, you can locate the names of those you interact with under the "players met" menu.
  • If you nonetheless decide to dismiss a summon after they aided you with a difficult part of a level you were stuck on, or because you are out of Estus flasks and need to use a bonfire, a common convention is to drop a thank you item for the summon before dismissing them. Typical rewards include a Humanity item (to repay the point of Humanity the summon would have received by helping you defeat the area boss), or a piece of titanite.
  • Most summons will know the area well and will be generally familiar with the common desires of a host, such as looting certain (or all) items, chasing down crystal lizards, or cutting off a boss's tail. If you as the host want anything different, it is incumbent upon you to make your desires known to those you summon. They will try their best to accomodate you and generally follow your lead, but are not mind readers.
  • If you get invaded and are near the boss fog gate, go through it. If one or more of your summons peels off at a chokepoint along the way, they are trying to buy you time to make it to the boss gate, which will send the invader home. They are not hoping you will stick around to watch a 1v1 duel, so move it.
  • If you get invaded and are not near the boss gate, let your summons take the lead. Many if not most invaders have created specific PvP builds and repeatedly invade particular areas that they like and know very well, so you should be wary of getting either separated from or bunched up with your summons and dying in an ambush. Learning how to space yourself is an important acquired skill; ideally you should stay close enough to your summons to quickly assist or be assisted when the invader appears, but not so close that you are all vulnerable to the same ambush or spell attack. As a host, never ever rush an invader unless they are already in trouble with your summons or you have no other choice; a seeming retreat by an invader is often just a trick to separate you from your summons, lure you into the area's enemies, or trigger another ambush. If the invader really does run away, use the time to heal your own summons or summon more. If you survive, you can always re-summon people you lost, but if you die then everyone loses.
  • You should also try to heal your summons with your own Estus flasks when appropriate, unless you are in a traditional PvP area such as the Kiln of the First Flame or Darkroot where unwritten rules to the contrary may exist (although even then it is still usually just a matter of discretion). Drinking one of your own Estus flasks also heals your summons for a reason: although many summons will bring their own healing items or spells for use in PvE when they can safely back off from computer opponents and heal, you as the host will be their primary source of healing in the heat of a PvP battle with a human invader. Do not be stingy with your Estus flasks if your summons are taking damage from an invader; there's no point in worrying about saving your flasks for the boss fight if you end up dying to invaders before you even get there. It is also usually fairly easy for an experienced player to heal during boss battles by using spells or Humanity items, making it entirely possible to defeat bosses even with no Estus flasks left.
  • Praise the Sun!

Now on to the boss breakdowns:

Each boss has been rated with an efficiency rating that is loosely based off of a combination of how much time you can expect to be waiting between summons, level conditions leading up to the boss, and the boss fight itself. These ratings are not set in stone and may vary greatly depending on your location and what time you play, but generally the higher the efficiency rating then the higher your chances of beating the boss for co-op rewards. Assume for these ratings that you are playing when traffic is likely to be highest. The ratings are as follows:

0: Never going to be summoned unless it is prearranged with a friend.
1: Summons may happen, but they are very rare. No specific time range, because too little co-op takes place in this area for any to be measured.
2: Summons do happen, but you’re going to have to wait. Some people may want help with this boss, but poor co-op conditions of the level or lack of a reliable summon sign location make these boss areas unreliable for successful summons.
3: The level and the boss fight are bearable, but the maximum of most of our patience levels fits here. Expect wait times of up to 20 minutes.
4: Expect wait times of up to 15 minutes, though faster is not unheard of. Take these areas to do your laundry/homework/pick up your room/etc. You’ll get some breather time, but don’t go too far away.
5: You will get summoned within as little as 1-10 minutes. High traffic and highly assistance-worthy bosses ensure quick summons.

If your level is at least 10 levels higher than the recommended levels, expect the efficiency to be that of a level 2, any higher and expect efficiencies of level 1 only.
Important Note: an important exception to these general guidelines is that many players finish their "builds" at level 120, and continue playing into NG+ and beyond. As a result, characters at or around level 120 are found in all areas of the game, and summoning at that level can be frequent anywhere. The below guidelines are most relevant to players still in NG.

Asylum Demon, Stray Demon:
Summoning Efficiency: N/A
Recommended Level: N/A
Pros: None
Cons: It is impossible to even drop a summon sign in the Northern Undead Asylum, therefore making it impossible to be summoned.

Taurus Demon:
Summoning Efficiency: 4 (0 above level 150)
Recommended Level: 5-10
Pros: As the Undead Burg is usually the first area players try to beat after leaving the Asylum, you are more likely to encounter new players as hosts who definitely need help, which can feel rewarding. Since there is no summoning in the starting Asylum area, this is also often the first time the host has seen summoning signs and summoned someone, so don't expect them to necessarily know anything about the level or even what your role as a summon is. This area also has an increasing number of low level but well outfitted invaders, which can make for a lot of fun and give the host a fast introduction to PvP. The most popular place for both summoning signs and invasions is outside the bonfire near the catwalk with the Hollows throwing firebombs.
Cons: The Undead Burg is a PvP hotspot, so be aware that not all hosts summoning in this area are doing so for boss runs. Summoning for PvE in this area drops pretty quickly beyond soul level 15. Just stay a bit underlevelled until you beat the Taurus Demon yourself, and talk to Solaire afterwards to get your White Sign Soapstone, then return to the Undead Burg and place your sign. Also expect inexperienced hosts to find all sorts of interesting ways to get themselves killed along the way, leading to a higher than usual number of environmental deaths compared to other areas. Many invaders will have pushed fairly far into the game before returning here, so it is also to your benefit to do things such as get an upgraded pyromancy glove and higher strength pyromancies to even the odds.

Bell Gargoyles:
Summoning Efficiency: 5 (2 above level 150)
Recommended Level: 15-25
Pros: Incredibly simple boss, lots of hosts wanting help with them, easiest place to be summoned for early entry to the Warriors of Sunlight.
Cons: Tricky summon sign locations. Since there are no bonfires that are actually nearby, you can’t follow the general rule very well. The best locations for this area are at the bottom of the stairs leading into the church just outside the bridge to Blacksmith Andrei’s Tower where you fight 3 hollowed soldiers, and at the bottom of the stairs inside the church next to the elevator shortcut back to Firelink Shrine. A third option is on the main stairs leading up to the armored boar, past the bonfire near the Sunlight Altar. If summoned there, be prepared to guide the host down under the alternate route if the portcullis behind the boar is down. Finally, one can also place signs at the doorway leading out under the bridge, to pick up hosts arriving either from the Burg bonfire (if the ladder is down) or who are trying to avoid the bridge wyvern.

Capra Demon:
Summoning Efficiency: 4 (2 above level 150)
Recommended Level: 20-30
Pros: Lots and lots of people want help with this boss, and the short length of the preceding level and relatively quick boss fight can make for frequent summons.
Cons: Odd summon sign location, the current best being on the small bridge right before his area gate, making this one of only 2 bosses you can acceptably put your sign right in front of the arena. The other preferred location is at the very beginning of the area, before the stairs down leading to the undead hounds and large bonfire. Invasions are common, so expect PvP. The fight itself is sometimes difficult to co-op as the Capra Demon and 2 dogs can swarm the host before the summons have time to follow through the fog gate, and a phantom sometimes gets in the way of the host due to the small boss area. If you are using a heavy weapon, try to stagger the Capra Demon as soon as you enter to buy the host time to deal with the dogs or get up the stairs to a safer location.

Gaping Dragon:
Summoning Efficiency: 4 (2 above level 150)
Recommended Level: 25-35
Pros: Easy fight, if long due to its high health. Going against the general rule of summon sign location, it is also fairly common to place signs right at the beginning of the area on or around the landing before the stairs leading down to the Hollows/Butchers, as many players come directly to the Depths in Human form after beating the Capra Demon with the help of summons. A summon who helps defeat both the invading NPC Darkwraith Kirk and the Gaping Dragon can earn upwards of 20,000 souls for what is often just a few minutes of work once summoned. The ability to farm Humanity items, various Titanite shards, and Eyes of Death from the rats, slimes, and basilisks respectively while waiting to be summoned also helps keep this typically optional area popular.
Cons: Depending on how much the host has cleared, the area before can be a breeze, or frustrating. If they haven’t already opened the shortcut, expect a cramped and directionless run through the Depths. If you can, try to be a guide if the host doesn’t seem to know where to go or gets lost. In particular, learn how to quickly get down into the Basilisk area from any place on the map, as hosts routinely fall into it from the open drains or waterfall. High curse resistance gear such as the Paladin Set or Antiquated Set (particularly the skirt) will enable you to rescue them quickly in such an event. Even if it looks like some of the area may have been cleared, on the way to the Gaping Dragon it is also worth a short detour down past the giant rats near the bottom of the shortcut stairway to ensure that Darkwraith Phantom Kirk invades (as this is the first of his 3 appearances, sometimes the host will not realize they didn't trigger it).

Chaos Witch Quelaag:
Summoning Efficiency: 5 (3 above level 150)
Recommended Level: 25-40
Pros: Simple fight with multiple people, HUGE amounts of souls for the level you fight at. The route from the nearest bonfire at the bottom of Blighttown to Quelaag's lair is one of the shortest and easiest in the game when done properly, and combined with the large amount of souls from the fight makes it possible to collect a great deal of souls and humanity in a short amount of time. This is particularly helpful for those who need both for Chaos Servant and/or pyromancy purposes.
Cons: The area before can get you if you aren’t prepared. Bring some Purple Moss Clumps so you don’t doom yourself with poison before even being summoned. It is very easy and quick to get to Queelag from the favored summoning area around the bonfire at the bottom of Blighttown, but if you notice the host picking up items along the way you should consider leading them on a full circuit of the area rather than just running straight to the boss. On occasion, the host may also have come in from the Valley of Drakes and wants to backtrack through the upper levels as well, so be familiar with the entire area.

Ceaseless Discharge:
Summoning Efficiency: 2 (1 above level 150)
Recommended Level: 40-45
Pros: Easy fight if you have a cooperative host.
Cons: Poor and often random summon sign locations; impossible fight if you don’t have a cooperative host. If the host is willing to sit back at the white light and do literally nothing while they let you exploit the various gimmicks of beating this guy, then it’s no worse than soloing him. If not, expect the boss to mess around outside of your attack range due to unstable aggro, OHKOing you and then the host (or the other way around).

Iron Golem:
Summoning Efficiency: 3 (2 above level 150)
Recommended Level: 40-50
Pros: Simple fight for good amount of souls, and mandatory boss and viable summoning area right before the boss fog gate means frequent summons.
Cons: Sen’s Fortress is a difficult area for co-op, this is one of only 2 bosses it is recommended that you put your sign right in front of the boss area, as the amount of tricks and player control required to get through the Fortress can actually be hindering to a host if a phantom is in their way of escaping a trap. If you prefer more of a challenge, it is not uncommon to place signs at the entrance to the fortress, although you will likely have to take a very active role in ensuring that various traps are avoided and that the host sees how to best navigate the swinging blades and other devices. Outside the fog gate to the Iron Golem, it is absolutely imperative of you as a summon to ensure that the Firebomb Giant up the adjacent stairs is killed, otherwise he will carpet bomb the boss fight and make your lives miserable. If you hear the explosions indicating the Giant is still alive and the host doesn't seem to be planning to go up and kill him, you should try and guide him there with an appropriate "Point Up!" gesture.

Crossbreed Priscilla:
Summoning Efficiency: 5 (3 above level 150)
Recommended Level: 55-60
Pros: If you know the area well and are willing to take a few shortcuts, getting to and beating the boss is a breeze. May also occasionally assist in killing NPC King Jeremiah, netting you even more souls than the boss itself.
Cons: Hosts love to explore this area. Expect every few hosts to wander around finding all the loot instead of going for the boss right away. Like the Depths, be ready to act as a guide if they seem lost. The tunnels and underground area filled with bonewheel skeletons are also notorious for surprising the experienced and inexperienced alike, so you may lose out on some boss runs due to the necessity of either seeing yourself slain to protect the host, or watching a host run from one wheel skeleton into a nest of other ones.

Dragonslayer Ornstein and Executioner Smough:
Summoning Efficiency: 5 (3 above level 150)
Recommended Level: 40-55
Pros: Easily one of the fastest summoning spots you’ll find in the game within the right levels, and due to the types of enemies and the boss itself, is also one of the best places to co-op for souls. Common summon spots include at the bottom of the main stairs leading down from the elevator and before the large rotating staircase, in and around the second bonfire past the two notorious silver knight archers, and in the area leading up to the boss gate for those coming in once the shortcut is opened.
Cons: Toughest level to assist someone through if they haven’t already opened the shortcut, as well as having the highest chance of getting invaded, so learn a bit of PvP just in case. Due to the amount of impact the order of killing them matters to the host, you will need to pay close attention to the host at the start of the fight on who they want to take out if they don’t message you out of game first, as hosts will not like it if you kill the wrong one first.

Dark Sun Gwyndolin:
Summoning Efficiency: 0
Recommended Level: 40-55
Pros: Relatively high amount of souls given per kill, large level range as people will take him on before or after beating Anor Londo.
Cons: It is impossible to know if the host is going for this boss or not, as the only summon sign locations even near him are on the bridge after leaving the rotating tower two levels above his area entrance, and these spots are the ones used to help people through Anor Londo towards Ornstein and Smough. You can indeed put your sign here and be summoned rather quickly, just don’t ever expect a random host to be running towards Gwyndolin.

Pinwheel:
Summoning Efficiency: 2 (0 above level 150)
Recommended Level: 20-40, 55-65
Pros: Arguably the easiest boss fight in the game, almost no special strategy required. Many players do not head to the Catacombs until they have the Lordvessel from Anor Londo and are planning to continue on to the Tomb of Giants, so it is not uncommon to be summoned well above level 40. PvP with invaders in the Catacombs can also be highly entertaining given the level design.
Cons: There is no good place to put your summon sign in the Catacombs, and very few people need help with him; this boss is commonly joked about with how people were able to kill it in 2 or 3 hits. Reaching him, however, can be more challenging due to enemies such as bonewheel skeletons, and it is possible to be summoned by leaving signs near the first or second bonfires in the catacombs, or on one of the main pathways.

Moonlight Butterfly:
Summoning Efficiency: 3 (1 above level 150)
Recommended Level: 15-25
Pros: Easy fight if you have ranged options.
Cons: Very few hosts actually need any help besides Witch Beatrice, so they don’t look for summons too much. In addition to Beatrice, it is possible to be summoned if you leave a sign near the entrance to the area, or on the stairs leading up to the boss gate.

Great Grey Wolf Sif:
Summoning Efficiency: 2
Recommended Level: 20-50
Pros: Easy fight if you can manage to stay between the boss's legs, huge level range as people will take on this boss at many points throughout the game.
Cons: Be aware that most people that may summon you here are probably going for Forest Hunter PvP; if it weren't for this fact then the summoning efficiency would be closer to level 4. Putting your summoning sign close to the doorway leading to Sif can significantly increase the odds of being summoned by a host planning to actually fight the boss, although at the cost of less hosts reaching the gate alive due to high volumes of Forest Hunter and other invaders.

Demon Firesage:
Summoning Efficiency: 4 (2 above level 150)
Recommended Level: 55-60
Pros: Short trip from the bonfire to the boss area, decent souls gained from the enemies on the way. A powerful and frustrating boss for many newer players, Demon Firesage can also be a challenge even for experienced summons and defeating him can be fun and gratifying for all involved.
Cons: His huge AoE attacks can be a pain to deal with if the host is unfamiliar with them and can’t dodge too well, be prepared to be hyper-aggressive to hold the boss's attention away from the host just in case.

Centipede Demon:
Summoning Efficiency: 5 (3 above level 150)
Recommended Level: 60+
Pros: The only boss to get reliable summons at unusual level ranges due to the reliance of the Orange Charred Ring in the fight; one of the shortest paths from the typical summon areas to the boss.
Cons: Takes a ring slot for the Orange Charred Ring, so be sure you’re build isn’t currently reliant on a 2-part set. At higher levels (about 90+), the efficiency lowers to be more around a level 4.

Bed of Chaos:
Summoning Efficiency: 2 (0 above level 150)
Recommended Level: 60-75
Pros: This is often a very annoying "boss"/minigame for many players, compounded by the challenging area and fairly common PvP invasions. As a result, if you are looking to do some good this area can be very rewarding, as hosts are extremely grateful for any assistance. Helping hosts learn how to quickly run from the bonfire to the boss fog gate can save them a lot of aggravation in the event of repeated deaths, and the PvP fights keep things interesting even if the ultimate boss battle is usually a study in frustration. The third invasion by Darkwraith Kirk occurs right near the fog boss gate, and generates a significant amount of souls for summons even if the party is later defeated by the boss. The most popular location for summon signs is near the second bonfire inside the small building, but being summoned at either the first bonfire past the Centipede Demon, or right outside the boss fog gate is also possible.
Cons: Fairly long run from the second bonfire to the boss room (although foes along the way can simply be avoided), and incredibly difficult fight to coordinate with someone who doesn’t know what they’re doing. Even if they do know what they’re doing, chance of success is relatively low, and isn’t boosted much by having a partner outside of breaking both orbs simultaneously. Hosts also often want to take two summons in with them, to maximize the odds against the boss. This can cause a fair amount of waiting at the bonfire while the host searches for other summons, and increases the likelihood of invasions.

Gravelord Nito:
Summoning Efficiency: 2 (1 above level 150)
Recommended Level: 60-75
Pros: Good amount of souls from his defeat, easy fight due to Nito's slow speed and weakness to pyromancies.
Cons: The Tomb of the Giants is a very frustrating area. The most popular location for summon signs is at the second bonfire, which at least ensures that the host has already had some experience with the various horribles in the Tomb of Giants in order to get that far. It is advisable to encourage the host to stick as close to you as they can and try to run your way through the remaining stretches, because if the host starts to freelance one or both of you will probably be killed.

Seath the Scaleless:
Summoning Efficiency: 4 (2 above level 150)
Recommended Level: 60-75
Pros: A good number of people want help to keep him distracted so they can cut off his tail. A popular location for summoning signs is at the final bonfire in the Duke's Archive, on the balcony. High volumes of PvP invasions can lead to fights with invaders in the Archives, the forest with the Crystal Golems, or all the way down into the Crystal Caves.
Cons: People who go after the tail tend to get smashed around by his tail’s attacks without realizing how tough they can be. Expect a few careless hosts eating these attacks and dying. Reaching Seath can also be a chore if the host is overaggressive with various foes, or is encountering the invisible pathways for the first time. This is one area where summons should always take the lead.

Four Kings:
Summoning Efficiency: 4 (2 above level 150)
Recommended Level: 60-75
Pros: Very fast summons as many people seek help with this fight. Summons happen frequently at nearly all levels, up to and including 120+.
Cons: Odd sign dropping location, the best spots currently being either in large open area with the distressed hollows, or on the circular platform after the first wooden plank bridge just past the distressed hollows. Given the nature of this fight, chances of success, even with help, without careful coordination is relatively low.

Gwyn, Lord of Cinder:
Summoning Efficiency: 5 (3 above level 150)
Recommended level: 65-80
Pros: Very wide level range since many people reach the end of the game at different times, one of the best bosses to co-op for soul farming, simple and short path from the summon location to the boss. Given that the area is rather short, this is a classic example of where the host can make a break for the boss gate to ensure they survive the frequent invasions that happen in this area. There is a fairly good chokepoint for summons to hold off invaders that pursue right where the 3rd Black Knight stands on the narrow walkways halfway through the level. Just be ready to sprint to the gate yourself as a single host may not last long against Gwyn.
Cons: The Kiln is a PvP hotspot, so there is not a great location for summoning signs as those left at the beginning of the level are usually triggered for PvP rather than boss runs. Placing a sign past the first Black Knight or at the boss fog gate can sometimes work to better ensure being summoned for PvE, though you will likely have to wait longer to be summoned if you do this. Be prepared to use your Black Separation Crystal if you are summoned for PvP instead of PvE. This also means that at the higher levels, where PvP is frequent, should you find a host actually doing a boss run, then invasions from phantoms will be frequent.

Sanctuary Guardian:
Summoning Efficiency: 4
Recommended level: 30-40, 55, 80+
Pros: Relatively easy fight if you know the combos and have some patience, but the fight is never boring. Summons may help with cutting off tail since openings are small. Only 1 area to drop signs. The Guardian's attacks, notably his melee combos and flying lightning sprays, can surprise and kill an unprepared host fairly easily and without warning, even if victory seemed imminent.
Cons: His combos drain a lot of stamina, he has several punishing area of effect attacks due to his lightning use and the water-filled zone, and also has the ability to poison you. High mobility makes him difficult to fight with no ranged options, and it can be difficult rescuing a host if they get into trouble due to a tendency for the Guardian to fixate on a particular target. Players enter this area at very different stages in their playthrough (although it does require the Lord Vessel and access to Duke's Archives), so the co-op sweet spot can sometimes be difficult to locate on a given day.

Artorias the Abysswalker:
Summoning Efficiency: 5
Recommended level: 40-60, 100-120
Pros: Very entertaining and challenging fight, and many people want help due to his aggressiveness. He is one of the more difficult bosses to defeat solo, so demand for co-op stays high. The best location to place your summon sign is right outside his fog gate, allowing for very fast repeat summons between fights. This is also the preferred location to be summoned for Kalameet, so it is generally a great location for co-op due to the variety of possible fights, and the high co-op demand for help with both bosses.
Cons: Many signs are placed in front of fog wall. For hosts, making it there alive may be challenging if you have not activated the shortcut, and many players play through the Royal Forest in hollow form to activate the elevators before returning in human form to summon assistance. For summons, this means you are unlikely to be called on to deal with PvP or any other portion of the level, which can sometimes get monotonous after a while (although you can also get summoned up near the Oolacile Sanctuary bridge if you prefer to run the full level). Artorias is also a dangerous opponent in NG and positively deadly in NG+ and beyond, so co-op wipes are fairly common. If the host does not stay on the defensive or otherwise is unfamiliar with Artorias' moveset it is likely to be a short and painful battle.

Manus, Father of the Abyss:
Summoning Efficiency: 5
Recommended level: 40-60, 80, 100-120
Pros: Extremely entertaining and challenging battle particularly on later playthroughs, and it is routine to be summoned all the way up in the Oolacile Township before entering the Chasm of the Abyss, so there is a lot of variety in the zone. Manus is a lot of fun to fight co-op, and is one of the few bosses that holds his own even in the face of three experienced players plus Sif (who can be summoned for the fight if you saved him before). In addition to the Township bonfire, it is fairly common to be summoned at the second bonfire at the entrance to the Chasm, or on one of the main paths in the area.
Cons: Powerful attacks that can take out a character quickly. Be sure you have a Silver Pendant to nullify his Dark Magic attacks, as they are very difficult to dodge. The Township area has an extremely high level of PvP invasions as well as a number of dangerous computer foes, so getting a host safely to Manus can be a challenge. The lower Chasm of the Abyss area has reduced PvP invasion rates, but most of the computer enemies there give little experience and can make getting through the zone a bit of a slog. The Township and Chasm are also one of the worst places in the game to be gravelorded due to the number and power of the black phantoms that appear (including multiple extremely dangerous Chained Prisoner black phantoms), creating either additional co-op dangers or challenges depending on one's appreciation level.

Black Dragon Kalameet:
Summoning Efficiency: 5
Recommended level: 40-60, 80, 100-120
Pros: One of the best fights in the game for co-op, with an impressive and sometimes overwhelming opponent as well as a large open arena. Hosts routinely need help cutting off his tail, so make sure to have a weapon or other attack that can do the job. High magic resist is recommended as Kalameet's fire does magic damage. It is possible to be summoned right outside Kalameet's fog gate in the lower valley, but the most popular spot by far is just outside Artorias's arena (Kalameet effectively becomes the new boss of the area once he is shot down), so players can enjoy fighting both Artorias and Kalameet by putting their sign in a single location.
Cons: One of the toughest bosses in the game due to high health and some surprising mobility. Especially dangerous to a summon since he can inflict Calamity status and you cannot heal through conventional means. The fight can last a long time particularly when Kalameet is not being accomodating with respect to letting his tail get cut off, or if he is spamming flame attacks. The large AoE of many of Kalameet's attacks, combined with his many knockdown abilities, can also make the battle frustrating and very dangerous at times.

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Now use this to summon one another as spirits, cross the gaps between the worlds, and engage in jolly co-operation. Praise the Sun!