you need to be around level 80, right when the battle starts hit triangle to block away and then immediatly attack, then use trinity limit over and over and then fly around to wait for the mp bar to go back up, and if he attacks you in a combo while flying, then hit square to counterattack, if you have retaliating slash equiped. then about half way through he will jump in the air and use that attack that takes you down to one hp one mp, and attack him before he does it and get a huge combo. but careful, he comes back quickly.