We've met a few people who'd like to go back and try their hand at the Coils of Bahamut. This is a great idea! That content is definitely worth seeing, and with a full level 60 party it's easy to breeze through at least the first two thirds.
Which brings me to the first point: if you've never done the Coils and you want to try them, get to 60 first and avoid the Duty Finder. You want to recruit a full party through PF so you can turn off the level sync. Ask for help with an "unsynced discovery party", mention that you've done your homework, and hopefully your PF will eventually fill. I'll join if I see it. It's a fun change of pace.
(Clarification: If you want to do it the synced way, that makes me very happy and I hope you have a great time. I'll help if there's anything I can do. Unfortunately I can't really recommend it to people any more because of how unlikely it is that they'd find 7 other players with enough time and interest.)
About that homework though, you don't want to use old level 50 guides. They are way more complicated than what you'll actually be doing, and learning them is a bigger investment than we're aiming for here. This is why I'm writing this post, I figured I'd try to write down the simplest possible spoiler-free strats that can get an okay-ish level 60 group through the Coils.
All turns can be solo-tanked unless noted otherwise.
Turn 1
Difficulty: Dragonskin Map.
ADS: Burn burn burn.
Caduceus: Burn burn burn. The boss will split in two. Keep burning.
Turn 2
Difficulty: Neverreap.
After killing the first miniboss, go either left or right, left is probably less annoying. Keep killing nodes until you reach the Defense node. The arena is tiny, but try to spread out as much as you can.
Someone will soon get a debuff called Allagan Rot. They'll need to run into someone else to pass it on before the timer reaches zero. Then that person will do the same when the counter runs low again, and so on. You won't have to do this more than one or two times. Actually you might even be able to kill the boss before the first debuff runs out? Please tell me about your experience.
Turn 3
Difficulty: YOLO
Just do it.
Turn 4
Difficulty: Dragonskin Map
Pull the things; AoE the things.
Turn 5
Difficulty: Alex 4 (story)
Put your Focus Target on Twintania. Don't ask, just do it.
Kill the adds, then watch what happens over your head. Red marker means share the damage with a friend. Blue marker means stay on your own and have the others kill the thing that traps you. Actually you'll probably skip before the first blue marker.
Twintania flies away. Spread out, try to find Twintania when she prepares to charge so you can dodge (or run into her, that seems to work well). Raise the people who got knocked into the wall. If there's no-one left to raise, try again, you'll have better luck next time. Snakes spawn, kill them, divebombs happen again, snakes spawn again.
When you're done with the snakes, go have a picnic in one of the the dark blue circles Twintania dropped (they're called Neurolinks) because she didn't expect you to kill the snakes so fast and she won't be back for a while. When she finally appears again, stay in the link until she does her big attack, then spread out in all the arena. You're now in phase 4 (where were the other three? Yeah I know).
Twisters are like normal circle AoEs (think Titan or A1), except there's no ground marker. And they one-shot you. Keep and eye on your focus target, and when Twin casts Twister, run in some kind of circle so you don't cross your own path nor anyone else's. People will die to this. Raise them. Knights will also spawn. Kill them.
At some point the twisters will stop, all that's left to do is finish the boss.
Turn 6 (Second Coil Turn 1)
Difficulty: Neverreap.
Tank the boss in the middle. Ignore the Dark Matter Bulbs except the ones that spawn in the middle, these need to die.
Break vines by running away from each other (the closer you were at the start, the easier, so try to keep a stacked formation by default). When someone gets a yellow mark on their head, try to stand on the other side of the boss so you don't get swallowed. Kill the bees.
At some point Rafflesia will cast Leaf Storm. This means no more yellow marks or bulbs. Instead you'll have more vines to break, and Dark Matter Honeys and Slugs will spawn. Leave the honeys alone, and let a healer tank the slugs (which you don't want to damage too much) while everyone keeps burning the boss.
Turn 7 (Second Coil Turn 2)
Difficulty: Neverreap.
Cursed Voice will put a triangle-shaped debuff on some people. When the timer reaches zero, they spit out a petrifying cone in front of them. Try to aim it in a direction that makes sense (out of the arena, away from teammates).
Renauds are unkillable unless petrified, and even then you don't really need to bother killing them. Really, just pretend they aren't there. They do take up a lot of space for something that isn't there, but at least you can't get more than four of them at the same time.
Kill every other type of add.
Cursed Shriek will put a circle-shaped debuff on someone, when the timer runs out the debuff will petrify everyone. That's okay, just wait it out.
Petrifaction, which is used by the last add and then by Melusine herself near the end, will petrify you if you're looking at the caster.
Turn 8 (Second Coil Turn 3)
Difficulty: A4 (story)
The boss never leaves the middle of the arena, and there are four half-circles on the ground along the edges, in each of these half-circles a tower can appear and do stuff. Put the "A" ground marker in the north circle, the "B" marker in the west circle, and the "C" marker in the south circle. Choose two ranged/healers, call them gofer 1 and gofer 2.
When a tower appears in A, gofer 1 will go in there and stay until they get one stack of debuff. When a tower appears in B, gofer 2 will go and stay for two stacks. When a tower appears in C, the main tank will go get three stacks.
(Note that sometimes you'll still have stacks when your tower spawns again; just go in anyways.)
Your basic formation should have everyone spread out (but stay out of the tower areas). When someone gets an orange circle under their feet, they're going to die; raise themyes that's how little fucks I give. The tank will keep the boss turned south and no-one should be in that part of the arena because ouch.
When an add spawns, kill it. When red landmines spawn, step on them to trigger them (Yes really. That's even part of the original strat), taking care not to get pushed into a tower field.
The boss will eventually cast Allagan Field on someone. You don't want that person to take too much damage. The healers should shield them, and if that person gets an orange tether to the boss someone else should grab it.
Turn 9 (Second Coil Turn 4)
Difficulty: Mechanics remain among the nastiest in the game, luckily you now have a lot more leeway with numbers. You can still go in with no experience and get your clear quickly, but only if a majority of your party knows the fight through and through.
There is very little that's cheeseable in this fight. I'll still try to simplify where I can, but this is mostly the same strat you'd use at level 50, meaning you can supplementing this with reading old 2.x guides.
Stay spread out in the first phase, avoid the center. The tank will explode once in a while so people should never stand too close. Red marker on your head after the boss jumps on someone means everyone should stack in the middle to share damage. Red or orange marker at any other time means a meteor is going to drop on you. Go to where you want to drop it (not too close to an ally or another meteor), wait until the marker disappears, and move out.
The boss will become invincible and three people will get green markers. Those are meteors as well; drop them away from each other and from the other meteors.
Golems will hatch from the green meteors; if you look closely they each glow with a different color. The tank will grab the green one, a DPS (or the offtank if you have one) will grab the red one, while the blue one will naturally go to the healers. All other DPS will focus down the green one first. Keep all three golems separate, and have them "eat" the meteors by moving them close. Make sure all meteors have been eaten before you kill the last golem.
After this, you'll get 6 meteors in quick succession, followed by three more golems. My suggestion is to spread out along the edge and drop meteors as they come, keeping an eye on everyone else so you don't accidentally stack two. You then drop the golems near the middle, in the centers of the three circles that the devs conveniently drew on the ground for you.
When you're done killing the new golems (same as before), stack up in a position a few meters away from the center (near the outer edge of the inner circles; you might want to put a ground marker here so you don't lose it when the ground drawings disappear later on.). The tank should stand separately because the boss will join back in by AoEing them. A giant-ass tower will fall in the middle of the arena. If you're under it, you'll die. If you're too far from the center, you'll get pushed into the wall and die.
After the tower disappears, the boss will call and add which you will kill (surprise!). When it dies, the add will drop small puddles of light. Don't go in there just yet. The people who get a purple (not red!) chain debuff will need to step in a puddle in order to clear it. The phase alternates towers and adds until the boss casts Bahamut's Favor, which signals the start of the last phase.
So yeah, phase 4.
The first important thing is fire. When someone gets an orange tether to one of the dragons flying outside the arena, that's a fire AoE coming to them. You'll need to alternate how you deal with fires. The first one is "out", meaning the target stays away and eats it alone. The second one is "in", meaning everyone stacks on the tank and eats it. Then out again, and in again. There are four fires per cycle.
(There are a number of possible complications with how fire works, but just know that someone can die mysteriously.)
Immediately after each fire comes a thunder. Someone will have a purple-ish debuff on them. That person needs to move away from the group so their debuff doesn't hit anyone when it explodes.
After the first fire in, people should start spreading around while leaving the middle of the arena free. They'll be conveniently spread out for the fire out. Someone will get a red marker and go to the middle. The others will wait until they've dropped two black AoEs on the outside of the arena, then run to the middle to help share damage. (Then it's the last fire in. Remember?)
After this, the other half of the phase 4 cycle is divebombs. You could probably skip it by killing the boss quickly enough, but since people will be dying left and right it most likely won't happen. Anyway, now's the time to have a side discussion about divebomb marking.
Turn 10 (Final Coil Turn 1)
Difficulty: A4 (story). Having two tanks is recommended.
The basic formation is spread out, avoiding the front (massive cleave) and back (massive tail cone) of the boss. Adds called Sons and Daughters of Imdugud will spawn at set points in the fight while Imdugud himself takes a break (meaning both tanks are free while you kill them). The tanks should each grab a pair of adds so Daughters stay away from other Daughters and Sons stay away from other Sons.
People will sometimes get red markers on their head with an assorted debuff. Healers should shield them so the followup attack does 0 damage. Scholars and nocturnal ASTs can use their AoE shield to cover everyone at once.
As the fight progresses you'll start seeing more and more of the actual main mechanic, which can be summarized like this: lightning will fall and hit a few people (with splash damage, that's why you stay spread out), giving them a vulnerability debuff. Then something will happen that requires people to share damage. If you were hit by lightning, stay away. If you weren't, go help.
There are two possible share-damage mechanics. Usually, someone will get a blue marker on their head, and place themselves some distance behind the boss. Able-bodied party members will stack between the boss and the target to absorb the incoming charge. The rarer mechanic is a lightning tether that'll tie someone to the boss. That person is going to explode in an AoE that must be shared with 1-2 others.
Once in a blue moon you'll see the tether and the charge at the same time, but with no lightning fall. In this case have 1-2 people help with the tether first, then everyone else help with the charge.
Turn 11
Difficulty: Bismarck EX. Two tanks are required here. Another vanilla turn where the strat is basically the same as level 50.
Place the three ground markers (A, B and C) around the center of the arena, in places that'd correspond to Kaliya's back (A), left side (B) and right side (C) if it were actually in the middle. It's not really there right now; that's okay, the markers will only matter in the phase 3.
The two tanks should always stand in front of the boss together, while the rest of the party will be in melee range (this is required in order to deal damage in the phase 3), preferably stacked neatly under the tail although this isn't necessary. Kaliya will deploy a field under itself that'll hit you with Heavy if you step inside its hitbox. This makes T11 one of the few fights where that target-lock feature thing is useful.
Blue AoEs will start as soon as the fight start. There's a "right" way to dodge them that'll become very important in phase 3, so get used to the dance now:
dodge the little cone to the left,
dodge the big AoEs to the closest side,
get back in position after each AoE and before the next.
When you see/hear Kaliya fire two shots into the air, look at your party members. If you see a target marker on someone, it's stack first, then spread. If the marker hasn't appeared yet, it's spread then stack.
Each cycle, Kaliya will stun the MT and use a skill called Secondary Head that'll give them a special debuff. If the follow-up attack hits them, they die. The OT should Provoke during the Secondary Head cast, that way the debuff will land on the MT but the attack will hit the OT. The MT can then Provoke again as soon as their stun wears off (or the two can swap roles until the next head, it doesn't make any difference).
The phase will end at 60% and Kaliya will run to the center (careful that it doesn't run you over), deploy an instant-kill field, and go AFK. Three adds will spawn: an egg-shaped one in the south, a cube in the northwest, and a sphere in the northeast. The egg will slowly move counterclockwise around the arena; the tanks need to each grab one of the other two adds and match the egg's movement (if two adds get too close to each other you're hosed). Meanwhile most of the party attacks the sphere (some people should stand close to it to help share some damage) while a caster and maybe another ranged attack the cube. The goal is to kill them at the same time (+- 10 seconds at most). A thunder debuff (the same as T9 phase 4) will randomly appear throughout this phase, so try not to get your mates paralyzed.
Once the sphere and cube are down everyone switches to the egg (you need to be next to it to deal damage), kills it, then everyone goes to A except the tanks who move to the opposite side.
Phase 3 is made up of mostly the same stuff as phase 1 (AoEs, spread/stack, secondary head) with one very big difference: tethers. Two people will be tied by a blue/black line. Two other people will be tied by a green/red line. If you're tethered and you move too far away from your partner, you'll stack vulnerabilities and eventually die. Same if you get too close to a member of the other pair. The two tanks can only ever be tied to each other, if you were worried about that.
When the tethers are being established, they'll appears between the boss and each target, instead of between the targets. Use this time to identify what's going on and react accordingly (it helps to spread out a bit so you can see who's actually tethered).
If the tanks have a tether, the other pair stay in A (i.e. behind the boss)
If the tanks don't have a tether, that means two pairs of DPS/healers. The pair with the green tether go to B, the pair with the blue one go to C (notice the colors match).
Everyone else should also be standing exactly on one of the markers. Yes, I know you're a melee and you want to be on the diagonal. I'm a monk too. But seriously, if this is your first time, just forget positionals and stay in place.
The little dance you learned comes in handy here. It allows you to always dodge in the same direction as your partner, and to never get too close to the other group.
The stack/spread mechanic needs to be handled a bit differently now. If you're tethered, just stay with your partner on spread, it's not a big deal. The people without tethers will need to move around on stack to ensure that the damage gets shared.
Turn 12 (Final Coil Turn 3)
Difficulty: Alex 4 (story). Two tanks are recommended.
Place the three ground markers east, south and west of the middle (close enough that melee can hit the boss in the middle from one of the marks, but no closer). The position is a bit finicky so leave it to an experienced player if you can.
MT pulls the boss to the middle, then stands to the north (where there's no marker). Swap tanks after each Revelation (tank buster, with a debuff that will kill you if you eat two in a row). If you only have one tank, that's okay, you just need enough DPS to reach phase 3 before the second Revelation.
Phase one: when the boss casts Blackfire, healers must stand in B, DPS in C and OT in A (or whatever arrangement your group uses; if you have no OT send in the MT instead). Stay on your marker until a black flame spawns under you, then move away (don't touch the flames after this). Some people will get white AoEs around them, use them to hit the flames and debuff them, then you can kill the flames with an AoE sneeze.
Kill the Bennu where it spawns. If you get a second one, it's cleaner to kill it away from the first one's corpse - but I'm not sure it matters at this point.
Phase two: have the two healers stand close to each other and away from the rest of the group. If you only have one healer, send a ranged DPS with them; I'll call them "the healers" anyway.
At the start of the phase someone will get a funky green-or-purple debuff; they need to pass that debuff to a healer by running into them (unless it's on a healer from the start; then awesome). The boss will regularly cast Flames of Unforgiveness, and every time he does the healers need to run into each other to swap the debuff during the cast. They'll take damage when the swap occurs, then again at the end of the cast - it's going to hurt a bit.
Meanwhile, someone will get a blue marker, and drop it some distance away from the boss but not too far either. It'll create a blue AoE that'll eventually condense into a small ice pool. The boss will then spawn a red fireball tethered to someone, that person should run into the pool, grab the buff, then run into the fireball. Everyone else should stay the hell away.
(Alternatively, an experienced player will volunteer to handle getting the buff and intercepting people's fireballs.)
The next phase starts when the boss goes invincible. The Bennus will come back to life, and then come back to life again after you kill them, and the whole thing will be kind of underwhelming because you'll only have 1-2 of them, 3 at most compared to the 4-5 the fight is tuned around.
Every time you kill a small Bennu, try to move the others away so they don't get buffed when it revives. Not that it's a big deal if you don't.
When you're done with the Bennus the boss will come back into the fight. Bring the green/purple debuff to the healers again, they'll handle it like in phase 2.
Ignore everything else, and finish the boss. If your DPS is slow you'll be treated to an unremarkable AoE phase and the cycle will rewind.
Turn 13 (Final Coil Turn 4)
Difficulty: Alex 4 (story). Two tanks are recommended.
Put the A marker right in the middle of the arena, it'll be useful.
The moves which the boss uses in phase 1 are actually used throughout the fight:
Megaflare is a cute little dance. Stand somewhere other than behind the boss, drop AoEs, move out of them, then the people with a white marker (all of them, and only them) meet on A to share damage. In later phases one or two towers will also spawn. Someone needs to stand in them so they don't explode.
Earthshaker targets two people who get markers on their heads. These people should each move to the edge on one side of the boss, drop their three puddles, and come back.
Gigaflare is used on phase transitions, and as part of the final phase cycle. It hits everyone and it kind of hurts.
Phase 2 begins with an add, which you kill where it stands. Orbs will spawn with a tether to someone; anyone caught in their blast will get a stack of stuff. Try to intercept orbs so that the stacks aren't all on the same person (2-3 is okay, more hurts). If your DPS is slow you might see a Megaflare in this phase too.
When the boss flies away, stack in the middle of the arena (conveniently there's an A there). The boss will reappear somewhere outside of the arena; home your camera in on him. When the red divebomb marker appears, do nothing. When the green marker appears, run straight toward the boss and pray that enough people in your party will be performing the actual strat instead. This is followed by a Megaflare, same deal as usual.
Wreck the adds as they spawn. The Pain of Meracydia (green) will switch between being immune to physical and magical damage; just wreck it too. After this is a second divebomb->megaflare phase (same deal) and more adds to wreck (yeah Twintania is an add now). Finally, the third and final divebomb->megaflare is a bit different because there's no green marker anymore. That was Twintania and she's indisposed now. Just get moving on the red marker instead.
After Megaflare, go chill out in the Neurolink, shield up, watch the fireworks and prepare for the final phase, which is similar to the first one with two key differences:
1).The boss now uses Akh Morn, the ultimate tank buster. If you have two tanks, they can take it together for manageable damage. If you only have one tank you'll need to grit your teeth here.
2).Every Megaflare and Earthshaker will come with a pair of tethers. The tethered people will cause a whirlwind, pushing everyone around them straight into the wall. It's easiest if the tanks can grab the tethers, but if not just deal with them intelligently.