Behemoth's are docile. They only counterattack. If your HP's low, stick to one powerful attack, and play defensively. Casting Holy or Meteor will cause them to counter with Maelstrom, knocking everyone's HP down to single digits.

Bahamut's a cakewalk. Just have FuSoYa and Rosa cast Reflect on everyone, and use your Lunar Curtains (Be conservative with them. You'll need them later on). He does more damage to himself than you do.