If you're going to use Parivir Abilities, whether as a main class or a sub class... use Geomancy. You learn that from a Sage's Robe on a black mage. It changes how different elements affect enemies.
Absorb---> Immune---> 1/2 Damage ---> Normal ---> Weak

With Geomancy on, and all the different elemental attacks a Parivir gets, you can almost always get at least one which they will be weak too. In my party of level 40's, I kept a human back at level 20 waiting for Parivir and the abilities so I could level him. He's doing more damage with that combo then Luso is with Dual Wield 20 levels higher. The best suggestion is Paladin/Parivir w/Geomancy. Then you have most elements at your disposal between the two classes, give them a fully powered Sequencer, and you have the strongest single hit attack in the game... can be better than Ultima, and it's every turn.

My personal favorite Combos:
Human:
Paladin/Parivir w/Geomancy
Paladin/Seer w/Dual Wield or Heritor/Seer (Adelle only) w/Dual Wield (Magick frenzy ftw)
Bangaa:
Master Monk/White Monk W/Shieldbearer. Equip no weapon and two shields. Get very high evasion.
Trickster/Gladiator w/ Unscarred. Very long range Ultima
Moogle:
Fusilier/Moogle Knight or Fusilier/Juggler w/Geomancy (for elemental bullets). One for good SE's and long range Ultima. The other for lots of good support
Viera:
Red Mage/Summoner W/Blood Price. Two summons every turn? Hell yes! (equip an elemental absorb robe, and hit yourself when you hit enemies, and it'll be free too)
Sniper/Spellblade... Long range SE application
Sniper/Assassin w/Blood Price. Long range Ultima every turn
Nu Mou:
Sage/Black Mage... widest variety of elemental attacks to use.. really the only Nu Mou I like.
Seeq:
Personally don't like them. But Ranger + Mirror Item + Item Lore + Xpotion = 100% 400 damage anytime.
Gria:
Hunter/Ravager... Sneak Attack from behind for massive damage at range, plus ultima.
Raptor/Geomancer... mostly support. Raptor has the best magic attack growth of the Gria, with good support abilities all around. Coupled with the situational magic of Geomancer... pretty good combo.

My final party?
Human: Paladin/Parivir
Bangaa: Master Monk/White Monk
Moogle: Fusilier/Mog Knight
Viera: Summoner/Red Mage
Gria: Hunter/Ravager
Last spot rotates depending on laws and such, but most commonly:
Nu Mou: Sage/Black Mage

THE JACKEL