i would level up. i barely remember this fight, so i cant give you much advice other than leveling up. also, try to economize damage as much as possible. if its yunas turn and you have to summon the aeon, but its still going to be some1's turn before he uses mana beam or whatever, take yuna out of the party for a strogner guy, have them attack, then put yuna back in on the otehr guys turn and summon the aeon. use all the little tiny advantages that you can.