The Don Tonberry trick.

Unlocked Don Tonberry in the Monster Arena. Get 2 of your strongest character, whom had killed most of the fiends in your adventure. (Most probably Tidus and Wakka/Auron). The third character is the one you want to level up.

Equip the third character with the ability Overdrive-AP. Then set his OD mode to comrade. When the battle starts, 2 of your strongest character attacks, thus he will get killed (or severely injured) by Don's Karma attack. Then the 2nd one attack, when the third character's turn (the one you wanna level up), just revive/heal and repeat the process. It helps if the weapon also has the double/triple OD.