That is the big creature with the unresistable stone effect right? If I recall correctly:

Use a gunner: Highroad Winds GG+Speed Bracer+Catnip. That's your damage dealer. You then need two other characters. Unlike most fights though, they need to act VERY FAST. Chac will turn a stoned character to dust if you don't heal them fast enough. If I recall, and I'm not saying I'm 100% accurate, but if you keep your characters unstoned he won't use any real attacks. (Someone with a better memory then mine please chime in.) The fight should be over fairly quickly, your biggest challange is keeping your characters alive and pink and fleshy.