I played through a significant portion of the game with Yuna, Rikku, and Paine using only the Trainer job class.
I believe in the power of humanity.
In the late game I tend to use either two Samurai and an Alchemist, or Yuna and Paine with Mascot with a garment grid and accessories to BDL and Rikku as an Alchemist, depending on if I've got Mascot at the time.
Three Dark Knights.
Have each one equiped with Haste Bangle and Ragnarok. Three Darkness attacks every round quickly eliminates almost any beastie you run across.
Sleepy Shadow has spoken.