To get AP just
Equip everyone with Comrade, Overdrive to AP weapons (triple overdrive, and triple AP are also nice for the weapons). Then either face
A. Don Tonberry
or
B. Cactuar King.

After getting the beaten out of you either flee, or kill the monster. All of the damage will have turned into AP easily getting you tons of AP in 1 battle.
If you don't want to get Don Tonberry then

A. Get a character loner (Just have them fight while the other characters have ran away or are dead)
B. Equip said overdrive to AP Weapon
C. Fight a magic urn in The Cavern of the Stolen Fayth
D. Press triangle a couple hundred times.
E. Flee