Originally posted by Psychotic
The way I level up is as follows

Equip Wakka with a weapon with:
Overdrive > AP (ESSENTIAL)
Triple AP (Or double AP, but triple is preferable)
Triple Overdrive (Or double Overdrive, but again, triple is preferable)

Then, switch his overdrive mode to comrade.

Next, choose to fight Don Tonberry in the monster arena. Then make the other two characters you've used throughout (Probably Tidus and someone else) and make one of those two, or both, attack Don Tonberry. Don will counter with Karma, which will do a lot of damage, and maybe even killing them. Wakka should revive these characters, until Don gets up close, then use the ability, flee.

You should gain a lot of Sphere levels this way, as I certainly have. It's also best to make sure your characters have auto-haste, or at least cast hastega at the start of the battle, so you can attack Don more times.
If you want to use this strategy the Cactuar King will do 99,999 damage a lot of the time, all of his attacks target one person (so you can revive easy) and eventually he'll run away, so it's an even easier battle.

And if you want to actually fight battles, the Omega Dungeon is your best bet.