quote canderson
Eh, I don't think it's worth leveling up to that point.
As for the topic, there's a Dream Eater, I forget it's name, but it has a skill called 'EXP Boost' that gives you more than the usual amount of EXP points after defeating enemies. Of course, the best place to train would be the final World, due to the enemies in that location giving the most EXP points.
If I'm correct, the EXP Boost you're mentioning only takes effect when you're at 25% of you're health or below, which really sucks.