Was pretty easy for me, did it on level 55 just after completing chocographs at start of disk 4 (i.e, no leveling up, had not entered remoria yet).

Importantly had completed friendly creatures sidequest.

Used Steiner, Vivi, Garnet and Zidane. Most important ability is clear headedness and then use remedies after curse. Alos, all characters had auto-regen. All characters equpped with shadow absorb (2 pumice, some type of vest for zidane and armour for steiner (uhh cantg remember what called, press select when choosing armour and it will tell you when gives absorb shadow)). Equpped Vivi with return magic. 2 reasons - when he casts level 3 holy vivi's recast does no damage but gives time for auto-regen to heal. Alos when he casts doomsday heals my entire part and does 5000 to himself. Then vivi's recast does 9999 to Ozma. as only has ~60000 health this is 1/4, and i had nothing to do with it.

Steiner shocked unless mini which meant remedy.

Zidane stole a few thing (has dark matter/cloak of lords/elixr/pumice piece) and then used remedies/phoenixes. unless dyne. Only had to use scoop art to get 9999 dmg so didnt waste mana on others. Attacked if nothing better to do.

Vivi cast flare, or used remedies and phoenixes.

Garnet kinda stood there, or attacked with tger racket. Used remedies, cast life.

Auto regen meant never really had to heal. only died occasionaly from meteor, took 4-5 tries to have good luch with meteor. A few chs had auto life to save first meteor cast.

All in all an unrewarding battle, easily one. Extreme dissapointment as this was hardest battle in game, took very little prep, only a few tries, no dramatic or poetic backstory like weapons in FFVII, and had correspondingly weak rewards.

Oh yeah, all auto-haste. I invested in giving everyone this ability as soon as i got feather boots (somewhere in disk 3). And low levels helped not get wooted by flatre stare

Ended up killing himself with doomsday

Good luck