First time I played FFXII, I tried to balance out my party, but then I ended up so underleveled at bosses that I struggled with a lot of them. Around Mt. Bur-Omisace, I decided I would just stick with a Balthier/Ashe/Basch team exclusively, and it worked out pretty well. Didn't really have any problems from that point forward.
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"Why must we always choose between certain death and probable death?"
"I'm in an epic battle to the death, FIGURE IT OUT YOURSELF!"