There's a lot of luck involved. If he does curse-meteor-curse, you're screwed. If he uses Doomsday though you are in good shape (you should use armor that absorbs dark). I can beat him pretty easily with Zidane (Attack), Aramant (Aura/attack), Quina (Frog Drop) and Eiko (Madeen/Cure depending on what side is showing), but it usually takes a long time because I like to steal. Garnet with Ark is a good choice too, with Auto-Regen her boosted summon can refill most of your HP.

EDIT: I thought this thread was for Ozma! Oh well, my strategy is same for just about any big fight: Zidane attack, Aramant Aura/attack, Quina Frog Drop, Eiko Holy/Heal.