I haven't done guides for the other coils, but figure to put this one up for the benefit of the current group that's starting t5, and others down the road.
traditional 2 tank, 2 healer, 4 dps. It's highly recommended to have WHM and SCH healers, and a mage. Not required, just recommended, and strategy below will assume you have a mage and a SCH. Another recommendation is a PLD off-tank, which makes the 4th phase easier.
You'll be required to be average item level 82. Especially for dps, more is better. Any physical attacker and tanks need to have 482 accuracy, mages need 435.
This fight is all about coordination and awareness. It really does help a lot to have a voice chat going. Without it you really need 8 people on top of everything. Especially for learning, do use some sort of voice chat so people can say what they're doing, call out what's coming, etc.
I'll just mention a couple mechanics here, otherwise the rest are mainly phase-unique and will save it those for the respective phases.
First, after engaging, the outer perimeter gets a purple-ish banner around it. If you touch this you take around 15k dmg and die.
Second, the main tank has to deal with something call death sentence throughout the entire fight. I don't understand how it works as I haven't tanked it, but I know steps need to be taken to reduce its impact. If ignored it'll kill the tank, so take actions like properly timing application of virus, stone/aldo, and mitigations. I believe it also puts a healing debuff on the tank as well. If anybody with experience on dealing with this can chime in, please do so and I'll update.
Another ability twintania uses throughout is plummet, which will do heavy damage to anybody who happens to get in front of her.
positions here will be relative to how you enter the room. the "back" will be 12 o'clock and where you enter is 6.
Oh, also everybody set Twintania as their focus target. This is critical later
twintania with 3 smaller dragons. Have the off tank number the 3 little guys 1-2-3. Order doesn't matter, but dps should kill in order and you don't want all 3 of them hanging around for long. The off tank gathers up everybody and shifts them slightly left, toward 9-10 o'clock. The main tank will then pull off twintania and pull her to about the 2-2:30 position, about 3/4 of the distance between the center and outer ring, and keep her facing outward.
The little guys spit an aoe called liquid hell. You won't see anything until the deed is done. If you're hit, move fast. The initial damage is light by the damage over time is severe if you don't move out. After burning the little guys, get to work on twintania. Prepare for positioning by having the off tank and Eos (and smn pet too) stacking by twintania's right leg. Everybody else get close, but not completely stacked, by her left leg. At 85% a mainly green circle (nurolink) appears under her (first it looks like a couple claws coming out of the ground, then a circle appears around them).
Phase 2 (fireballs and conflagurations)
Fireball and conflaguration are tied to one another.
Fireball - red triangle over the target's head, the impact is a small aoe where damage is shared by the number of players that get hit. The number of players hit will determine how long the next conflaguration will last. If 3 players are hit, 20 seconds. If 4 are hit, 15 seconds.
Conflaguration - blue circle over the target's head. This player must quickly get directly under twintania. The result is a jail the same size as twintania's hit box, so everybody steer clear. But anybody inside a conflag will not take damage. Break this before the (invisible) timer expires so the jailed player(s) don't die.
Now for the actual action... as soon as that nurolink is dropped, a player is targeted by fireball. That player alone moves over to the off tank and pets stack, so all 3 or 4 absorb the damage. This is a good time for the WHM to drop a cure3 on the target for fast recovery.
When that first fireball drops, the main tank will move twintania down to around the 4:30-5 o'clock position, close to a notable "valley" along the outer ring, and keep her facing in the direction she moved. This valley is important for the next phase. Maintain the same stack of off tank and pets by her back right leg, and everybody else hanging around her left side, very close to her but not stacked, so you can see the fireball or conflag target you. If you get a conflag, run directly to the center of twintania's hit box. dps work these conflags down quickly, but do not burn cooldowns here. Healers and tanks help too, especially if you get the scenario where 2 dps are within a conflag.
If, while a conflag is up, a fireball targets you, just run into the conflag. If no conflags are up, then run over to the off tank and pet stack. fireballs happen roughly twice as often as conflags.
This phase keeps going until she's down to 55%. a 2nd nurolink drops, and get ready for the toughest part of the fight.
Phase 3 (divebombs and snakes)
Divebomb - a green circle appears above the target. The aoe itself is a circle around the target then linear from left to right on the target, relative to where the target is facing. If hit you take around 3000 dmg and get knocked across half the distance of the arena.
How to deal with divebombs? The entire party stacks against the purple wall in the center of the pit. When the chime and green circle appear, everybody run out of the pit and close to a line between the 2 nurolinks, then run back down into the pit. Do this 3 times. After the 3rd executes, it will be 45 seconds until another set of 3 will come. If you get hit by a divebomb, stay out in the arena, don't readily return to the party. You can move somewhat close though, and should so eos can heal you. Again, face toward or away from the party, never have them to your side! If the party gets targeted with the next divebomb, go ahead and meet them above the pit as they're dodging. If you get targeted, move toward or away from the party to dodge, but ideally stay behind a line that can be drawn between the 2 nurolinks. If you're close by you can rejoin the party before the next divebomb, otherwise continue to stay away.
Now for the actual action...
Immediately when the 2nd nurolink drops, everybody get into the aforementioned valley against the purple ring. Leave the pets where they are. All 8 players stack perfectly in the base of that valley against the purple ring. When the green circle appears, everybody dodge as described above, and return to original position. On the 3rd divebomb, 3 snakes will spawn. Watch the time, in 45 seconds 3 more divebombs are coming.
When the snakes spawn, the off tank grabs both hygeia, and the main tank grab the big snake, and everybody get back into the base of the valley. Work on the Hygeia snakes, wear both of them down to 40-50% then get to work on Asceplius. Ideally, a blm alone can aoe down the snakes, and everybody else focus on Asceplius. DPS in this phase gets critical, do your best damage without burning cooldowns. Be on your toes for the green circle to signify divebombs again. The 2nd set of divebombs can really catch you off guard. If someone can be a time keeper on voice chat, or have a macro countdown (be sure to start it when the 3rd divebomb of the 1st set is executed), it'll help the party.
Dodge the 2nd set of divebombs just like the first set, don't mind the snakes. After the 2nd the off tank can keep running to the opposite side of the room (would inherently dodge a divebomb if targeted anyway) and pull the 2 more Hygeia that spawn (they spawn against the perimeter at roughly 8-9 o'clock)
Now for the most critical part of the fight. The off tank will gather all 4 hygeia and the main tank has Asceplius. The off tank should bring the 4 hygeia close to the back side of Asceplius. The main tank is then in front of Asceplius. Get in this position as quickly as possible, and have the BLM or SMN do limit break. At this time melee dps should not be in the action until the weakened hygeias are dead! Do not move from positions, all dps need to kill the remaining hygeia very quickly. Melee need to back away when a hygeia is about to die, to avoid getting the defensive debuff. As soon as Asceplius has all 4 stacks of the debuff (and hopefully everything stayed in position such that the main tank has 0), the main tank will move over by the first nurolink, and ranged/healers get by the 2nd nurolink. Now is the time for all out epic dps. Burn cooldowns, use potions, properly use debuffs that help other classes, and work your rotations to their max potential, as Asceplius needs to die before twintania returns. DO NOT stand in the link as it will cut your damage in half! Stay nearby though...
The total time for the previous paragraph to get executed is roughly 60 seconds. If the snake is still alive after twintania's return, a wipe is likely.
Upon Twin's return (it's very obvious), the main tank will go into the 1st nurolink (should be right next to her), and everybody else step into the 2nd nurolink. If the off tank still has debuff stacks, make sure they're topped off and pop a mitigation, stone/aldo help too. Of course, make sure the main tank is topped off too as they will potentially eat 3 attacks in short order (blast, plummet, death sentence). As long as nobody else received the debuff from a Hygeia's death, everybody will take around 3000 dmg from the blast if they're within a nurolink (dead otherwise).
Phase 4 (twisters and dreadknights)
The worst is done, but still this is a tough phase as you really need to be paying attention. If you lose focus you can easily end up dead and start a wipe.
Twister - an invisible circular aoe that targets 4 players. To dodge this, be constantly moving, run in a small circle and be sure not to overlap movement with anybody else. Therefore it's important to be scattered around during this phase. If you're hit, you're dead. If everybody dodges accordingly, a small green twister is left on the floor for a few seconds, don't walk into it.
Dreadknight - an add that spawns in the center, targets 1 player and jails them. The dread then moves to that player, and if it makes contact the player is dead. Any stun abilities help (PLD will shine here), lethargy and blizzard 1 are extremely helpful. Goes without saying, dps need to kill these things fast.
After Twintania's blast, the main tank can keep her where she is for now, especially if it helps people see the dreads that will spawn dead center. Before 30% health though, the main tank must put twintania in the center of the arena. Everybody else spread out. The first twister comes rather quickly, so be ready to move around to avoid that. Make sure for melees there's a good clear space for them to back away a bit, so the little twister isn't left right where they need to fight. Work the dreads when they appear, but watch that focus target for when Twin brings about twisters, it will happen while you're dealing with dreads. Twisters come up about twice as often as dreads.
Just be aware of twisters and systematically work the dreads when they appear, and of course work twintania as hard as possible.
At 30% the 3rd and final nurolink is dropped. Nobody move really, just keep doing what you were doing but now relax as no more twisters are coming.
Phase 5 (hatches and liquid hell)
Everybody stay spread around still, but the off tank will park in the nurolink directly under Twintania. For the rest of the fight, their job is to absorb hatches. A triangle will surround the targeted player, and then an orb will travel (somewhat quickly) from twintania to the target, killing them on impact. But, since the off tank is in the exact spot where the orb will spawn, they eat the damage for you. If for some reason the off tank doesn't/can't get the orb (as an example, they may have been targeted with liquid hell and are placing it properly), then quickly get into the nearest nurolink and take the hit there to survive it. Liquid hell is just like what you saw with the small dragons at the start, except this time there will be 5 in succession (think like the eruptions in ultima hard or ifrit extreme). It's best these be placed along the outer perimeter, so if you get targeted, run (don't sprint, just run) to the outer perimeter if needed, then along the outer perimeter. Anybody else in the way move accordingly.
This phase isn't too hard, the mechanics are simple compared to what you've been through. Just keep your cool and cruise on to victory!
Oh 1 final thing... there's a 13 minute enrage timer from the initial pull, which I believe is impossible to survive.