As long as you have everyone above 2000 hp's, then there should be no way Big Bang can finish you off.

Anyhow, do this with the following people...

Kain: Jump. He'll miss Big Bang occasionally.
Rydia: Either summon Asura to help cure, or Bahamat to attack.
Cecil: Attack with the Ragnorak/Crystal Sword.
Rosa: Cure 4 every turn. If someone dies, get someone else to revive them.
Edge: Throw all your weapons at him. In particular, the excalibur and the spoon from Yang's wife.

Level 50-55 should be okay.