Disgaea 2: Cursed Memories

Disgaea tends to be set up where the stronger the character is, the faster they can power level. Ultimately, the best powerleveling in D2 is Cave of Ordeals 4 + all Stronger Enemies Bills + 300 Felonies + Land of Carnage. Getting a level 1900 Armsmaster and grinding a character's weaponmastery (usually either Sword or Staff) helps a lot with increasing power.

If monster you can use capturing trick. You can go to 4-3 where there are many neko with 10%level up geo symbols. you can capture with throwing the neko into base panel. but be careful! don't use it so much more than 2x of highest level of your strongest character in base panel. If human, go to cave of ordeals 4 with all Stronger Enemies Bills, and use it in land of carnage.(It's easier when you have the nekos