Getting AP by fighting normal battles will take a long time. This trick essentially abuses the nature of the AP and Overdrive - AP systems so that you can gain AP en masse in single battles, maybe even enough to max out your Sphere Level at lower levels! You will need the following:
- Overdrive Mode: Comrade or Stoic
- Weapon Abilities: Overdrive - AP, Triple Overdrive (Double can also work), Triple/Double AP
- Armor Abilities: Auto-Phoenix, Auto-Haste
Other things you will want to have or need is no HP-increasing abilities on your armor, as the lower your health is, the quicker the Overdrive will fill up. Also, to do this trick, you will need [Don Tonberry] unlocked in the Monster Arena, which is done by capturing at least one of every monster in the Cavern of the Stolen Fayth. Also, it’s a good idea to have the Flee command on every single character participating, which is near the beginning of Tidus’ portion in the Sphere Grid.
The strategy to earn a lot of AP is rather simple. Go into the Monster Arena and fight the Don Tonberry. Attack any way you want and he’ll counter with Karma which should deal insane damage to the attacker; 15,000+ is not uncommon. The attacker will likely die and, in the process, due to Comrade, their allies’ Overdrive gauges will charge. That Overdrive will get more because of Triple Overdrive, and that will convert to AP with Overdrive - AP, which is increased even more with Triple AP. Essentially, with a good enough hit against the attacker, per the mechanics of Overdrive - AP, that’s 9 Sphere Levels. Repeatedly Attack and let Auto-Phoenix do the reviving until the Don Tonberry gets close, and then Flee and sit there while a massive AP counter ticks down.