To help with your problem, i would also have to say Omega Ruins.

Not only does it have stronger monsters than the ones inside of sin, but they also give great ap!!!

Best way to gain AP? Don Tonberry...

No seriously. Put it this way, you can gain 99 Sphere Levels in a few minutes this way.

Armour Set up should have Auto-Phoenix on each character, or at least 2 characters. Auto-Haste is also a good idea. Have one character (the Attacker) in Stoic overdrive mode, the other two in Comrade mode.

Beat up One-Eye a few times until you have Triple Overdrive weapons with two free slots for each character. In those slots, customise Overdrive -> AP and Triple AP abilities. So that way you are gaining NINE TIMES! the number of AP.

Equip these AP gaining weapons to every character. Next get in battle with Don Tonberry. Have the two Comrade characters do nothing, and the Stoic character attacks. Auto-Phoenix
will revive them after Karma kills them, and all that damage will have gone straight into your AP, multiplied 9 times from the weapon. Karma does a lot of damage, so you can see that really building up your AP.

You don't have to win this fight either, just run away when Don Tonberry gets close so he doesn't use Everyone's Grudge. You will still keep all the AP gained, and you'll notice that the Stoic character should have a whole load of AP, and the other two characters a lot less, but still a lot.