Here are a couple of ideas, if you fight the one I think you fight. First of all, make sure that when you kill there is NOONE inside his chest cage, else he'll spit him/ her out and kill everyone. If you see that it has few HP left and there is someone trapped in it, then just miss his/ her turn (guard). Use Rikku to use Al-Bhed Remedy on everyone, or any other healing potions. As far as I can remember, the thing has around 30000 HP. When it goes bellow 10000, you better finish him with one move, with Wakka's overdrive perhaps. And you'll need armour that prevents you from turning into stone on Rikku, Tidus and Wakka. That's the way I did it, anyway. The key point is not to kill it when someone is trapped inside.