Before the Overdrive fills up, pull out an Aeon. After he uses the overdrive, take out Wakka, Lulu, and Yuna (because Auron, Kimahri, Tidus, and Rikku can't hit him). If you've got Ultima with Lulu and Yuna, use it!!! If you've got Grand Summon, use it!!! Use all the overdrives. If you've got Wakka's Auroch's Wheels overdrive, use it!!!

If you just summon Aeons before overdrive, you will win, easily.