I've now spent a grand total of 270 minutes trying to beat this boss. I seem to keep getting put in groups with apathetic healers that constantly die and just go "lol idk idk liek totally liek".
BE BETTER.
So I FINALLY beat her, WOOHOO!!!
OK, here's how the fight goes. You're all going to want to follow this to the letter to get through it, but it'll likely still take a lot of attempts.
The battle takes place inside a hurricane with 4 pillars. Standard buffing from healer, then everyone needs to approach her around the rim in a ring formation, that way she doesn't send wind attacks through the middle and knock down the pillars (which you need standing to live). She has an AOE attack that hits everything in front of her, and where a lot of tanks fail is that they go through her but take too long to get back to where she was facing, so she gets attacks off in the direction of the pillars - NO. This cannot be allowed to happen. If it happens with your tank, spank his/her ass.
The tank has to have her facing outward, towards the hurricane (and thus away from the pillars) - she'll still attack everyone else, so they need to place themselves away from all the pillars aswell.
Then she'll disappear and reappear in the exact same spot. Everyone needs to get behind the closest pillar and be completely out of her line of sight otherwise they'll die. If one person dies, it's nearly impossible to recover. Later in the fight it's completely impossible.
When she reappears, she'll summon feathers. The feathers will all go towards the pillars. If you have a summoner - and this bit is important - if you have a summoner, immediately cast Tri-Disaster on her. This binds the feathers to their position, so they can't go to the pillars. Meaning they explode where they are, doing no damage to the pillars. You just can't cast any AOE attacks or attack the feathers, but this allows you to completely ignore the feathers and focus on her.
If you don't have a summoner, the tank has to stay with her and the DPS's have to get the feathers. It's best to pick a pillar to protect the most (you need at least one fully standing pillar for later) and kill the feathers near that one first. The tank should also take special care that she's not aimed anywhere near specifically one or two pillars that you're protecting.
After the first feathers, attack her a couple of times and she'll disappear again. She'll reappear at the other end of the arena - hide behind a pillar. Here the tank should get to her as quickly as possible otherwise she'll make her way into the middle of the arena which will result in more damage to pillars. Then she'll summon more feathers, defeat as above.
Then she'll disappear and appear in the middle - except this time she just summons feathers, which you need to kill. When she's below 50% HP, she'll again disappear and reappear in the middle. This is when she casts the move that almost always kills at least one person, because a person is being stupid/ the pillars aren't intact. You need to stand behind the biggest pillar you have remaining when she says "THE RECKONING COMES" otherwise you'll die. The damage done depends on the number of rocks on the pillar you're standing behind. The healer cannot survive this attack unless they're standing behind a full pillar. This big hurricane move will destroy all the rocks and reduce everyone's HP to about 1/10th of what it was if you're standing behind a large pillar. It's unavoidable.
Immediately after this attack, the HEALER should use their limit break, because if they don't someone will die, and if someone dies, it's over. You need every move to count. She'll create a small ring in the middle of the arena, and if you step outside this ring, you'll die. If you stand in front of her, you'll die because she attack spams like mad. But the tank can't keep her facing in one direction at this point, because she just randomly spins around, so it becomes about trying really hard to keep running behind her and whatever you do, do not stop attacking. You need to keep attacking to get her HP down.
You can win at that point, but if it takes too long she has an ultimate move where she disappears then appears at another end of the arena. If you're not standing on the pixel that is as far away from her as possible (but remember you can't touch the outside of the ring she's made) then you'll die. When I landed the killing blow I was the last person alive with 43 HP remaining (out of 2,500 - and some of her attacks hit for 2k HP) but it needn't have been that close if the healer hadn't died. The healer has a really tough job btw, because they can't afford to tunnel-vision the HP bars or they'll die. That happened a lot. It's ALWAYS the healer that died that kills the chance, so if you get a healer that's done the fight before you're in with a better chance than most people.
Good luck, and thank fuck I don't have to do this fight again. Because it's the hardest FF boss ever.