1) Fight One-Eyes, which drop weapons with Triple AP. BDL weapons and Auto-Haste make this much easier. Repeat until you have enough weapons for the characters you want. Save the ones with two extra slots.

2) Customize them with Overdrive -> AP and Triple Overdrive. You'll now have super AP-gaining weapons!

3) Customize armor with at least Auto-Pheonix.

4) Set the battle party's Overdrives to Ally (Overdrive fills when allies take damage).

5) Take on Don Tonberry. He'll counterattack with one-hit kills, but only one character at a time and Auto-Pheonix should take care of it. The more damage his counterattacks do, the more AP the surviving allies get.