Comrade + Triple Overdrive + Overdrive->AP + Cactuar King or Don Tonberry = AP up the wazoo.

'Comrade' ability is the best Overdrive mode for gaining AP; generally, though, I use Stoic - the default mode where the Overdrive gauge fills when the character takes damage.

You gain new Overdrive modes depending on what happens to your team. When a character slaughters a lot of Fiends, you gain modes that raise your gauge whenever that character attacks. Taking a lot of Poison attacks will earn you the mode where your gauge increases when you experience a status abnormality... and so forth.