I've basically kept the same set-up for most of the game - I'm now tying up loose ends before hitting the Bahamut. In my party I have the following basic set-ups:

Thief/Tank/Party Leader (Vaan, Penelo)
Heavy Hitter (Basch, Ashe)
Ranged Fighter (Balthier, Fran)

Vaan and Penelo equip a sword/dagger and shield. Basche and Ashe equip 2H weapons and Balthier and Fran equip ranged weapons. I rotate each pairing so that the character at the lower level runs with the party.

All of the characters have the basic healing and quick-kill Gambits:

Self: <10% MP - Charge
Ally: Any - Raise
Ally: <20% HP - Curaga
Ally: <40% HP - Cura
Foe: HP = Critical - Attack
Ally: Any - Esuna

Penelo and Vaan have the following (and are equipped with Thiefs cuffs). They (ideally) attempt at least one steal from each mob, then go for the mob with the lowest HP. They only go for each mob once because the Heavy Hitters should knock each foe off 100% health after the steal:

Foe: HP = 100% - Steal
Foe: Flying = Flare (or another damage spell or Telekinesis)
Foe: Lowest HP - Attack

Ashe and Basch will run around and stop the Thief from redundantly stealing, then go for the enemy with the lowest HP. They also make sure that the party has Protect between fights.

Foe: Flying = Flare (or another damage spell or Telekinesis)
Foe: HP = 100% - Attack
Foe: Lowest HP - Attack
Ally: Any - Protectga

Balthier and Fran hang back and are usually the ones to bail the party out if they get in a mess. They also keep Libra and Shell up, and haste anyone who gets slowed.

Ally: Slow - Haste
Foe: Lowest HP - Attack
Ally: Any - Shellga
Self: Libra

Which leaves plenty of gambit slots for 'situational gambits'. For example, if lots of enemies are using a specific status effect, you can put in a cheaper healing spell above the Esuna gambit. Having Chronos Tear and Cleanse gambits helps where enemies use those status efects.

Using that rough set-up, I finished Ridorana in the high 30s, if I recall correctly.