Quote Originally Posted by abrojtm
If you have an AP gaining weapon, simply go fight Don Tonberry with Overdrive set as Comrade (or Stoic if you're the one getting hit). Don should hit you for 99,999 every time assuming you've defeated enough fiends with that character. 99 Sphere Levels per battle-easy.
I do that with my character, there levels went up to 55 by defeating 1 Don Tonberry.