Ozma's difficulty in a way that he usually starts the battle by casting Meteor, which usually kills almost everyone, unless they have Auto-Life. If that happens, I'd suggest resetting. Another dangerous attack he uses is Curse. It does a lot of damage and gives you many status ailments. Be sure you're protected against those. Ozma is also fast, I'd suggest casting Haste on everyone (I'm not sure if there was an ability called Auto-Haste, use that if you have it), and everyone should have Auto-Regen and Auto-Potion too. Make sure you're protected against Doomsday, even better if you can absorb it, so Ozma will just waste a turn healing your characters and possibly hurting himself. Have Eiko or Dagger heal.

If you want to deal physical attacks, do the quest of friendly monsters. I'd suggest fighting a lot of Dragons so Freya can use her Dragon Crest. It does 9999 damage even when she's under Mini status (Steiner's Climhazzard and Shock won't do 9999 when he's under Mini status). If you want to take a chance, you can have Vivi in your party and have him use Doomsday magic in Trance. Hopefully it will hurt Ozma and heal you, or you're in trouble. I never bothered stealing much with Zidane, so he's not that useful in the battle. I think I had him use a Dark Matter against Ozma, though, it does 9999 damage. Quina's Angel Snack is good, but I think Quina's too slow so I don't use him/her much. If you want, you can take Amarant to the battle when he's close to Trance, so when Ozma uses Curse, Amarant goes into Trance and gets the abilities to heal everyone, restore everyone's status etc.

Good luck. If there's two battles in the game where you'll need it, this is one of them (the other is the final battle since if Grand Cross does Death to 3 characters and Doom to 1, you're done for, and something like that has happened to me, like, twice).