If you encountered all the spirits in Gaia and satisfied their requests go to Chocobo Aerial Garden (you will need a Gold Chocobo and a Dead Pepper). Examine the stone on the second platform and choose the third option, then the first option to fight Ozma. To defeat Ozma, put Zidane, Dagger, Quina and Eiko in your party. Have Quina use Magic Hammer three or four times to steal all Ozma's Magic Points -- from now on it cannot use any spells. Then, have Zidane and Quina attack and Dagger and Eiko heal (do not use summons, you need a large amount of MP in this battle). With some of patience (an half of an hour if you are lucky), you should defeat Ozma.