Well, as far as I remember, most magic is useless against Belial... do you have some good white magic (cura, protect, shell)? You'll basically have to stick with physical attacks to do any real damage... Also, if you can get the quickenings for your main 3, I recommend doing so...
I also recommend switching one of your characters out for Fran and use the quickening at the beginning of the battle. Be sure to keep trying to reset and chain the quickening. Afterward, if Penelo or Balthier have any quickenings, switch Fran out for them (one at a time) and repeat. After you've used your quickenings, try to take him down with physical attacks, using healing spells, potions, and phoenix downs as necessary.