I'm on my third playthrough, currently at the Via Infinito.

Yuna: Gun Mage (100%), Iron Duke, Invincible. Supernova and Mortar deal 25-50,000 damage against any enemy and Drake Slayer against Elder Drakes deals 30,000 damage. (This is my favourite combination)

Rikku: Gunner (100%), Key to Success, Cat Nip. Double HP and MP, higher Luck. Cat Nip is self-explanitory.

Paine: Dark Knight (100%), Iron Duke, Ragnarok. Black Magic (GG dependent), Arcana and Darkness are all free to cast thanks to Ragnarok, and Black Sky hurts 5-6,000 per hit.