Start capturing monsters. You need to get weapons with:
Double/Triple Overdrive
Double/Triple AP
Overdrive -> AP
The Don Tonberry trick won't work otherwise (well, technically it'll work with JUST Overdrive->AP, but it will take forever). By capturing monsters, you should be able to make 6 of the weapons, provided you get 6 weapons by beating One-Eye in the Arena.
For Don Tonberry, he does more damage based on how many kills that character has, so whomever has the most kills, use them to attack Don Tonberry. Put that person on Stoic OD mode, and the other 2 on Comrade.