I based everything on the character's stats, and I only really used Vaan, Ashe, and Balthier, the other characters were just strong enough to revive the main party.

Vaan was a warrior, and was given the Zodiac Spear as soon as I got it. He learned Green Magic, but never really used it.

Ashe was a white mage, and used Katanas. She would be my primary white mage and, with her magic, she could deal some pretty good damage with the katanas.

Balthier had a bow, and used items and some white magic. While Ashe's gambits would be maybe Hp<70% Cura, his was HP<50% Hi-Potion, so we wouldn't get overwhelmed as easy. At some more hectic times, Ashe and Balthier were both healing constantly, while Vaan was the one doing all the damage(The Gilgamesh battle comes to mind).