My characters were all at level 20ish and I beat him. don't bother casting spells (except white magic) forget belias (Gil Snapper's resistant to fire and the damn thing attacked the wrong target anyway). Get a bunch of characters with quickenings (I got Vaan, Basch, Ashe and Penelo) and use the quickenings. Then, use an ether to refill the character with the lowest max MP (for me it was Basch with 56) and use him to start another mist chain. Repeat until victorious. Set aside hours of free time to fight this thing 'cause it'll take quite a few attempts. And stock plenty of Hi-potions and get some characters with a couple Ether Lore liscences equipped. Good luck.