Pick whoever you want really-- Ashe and Penelo have high natural magick, Balthier and Basch have high strength, and Fran and Vaan are pretty well rounded.

I used Vaan/Penelo/Ashe, but if I could do it over I'd probably do Balthier/Fran/Ashe.

As for the boss, he shouldn't be too hard if you land a good quickening chain/concurrence. If you still can't handle him, level up.