The Barrier Magic has a field skill "Field Barrier 3" which only costs 1 MP (instead of 10) at rank 50. Once you earn and assign this skill, you can venture to places with plenty of enemies you've defeated before and simply walk into them with the Black Belt and Eyepatch accessories (the skills Attract Aura and Regenerate MP at this point are self-explanatory). Some good places would be:

Underground River - the Kelolon Defender Room, all the way to Baroy Town (80 enemies)
Primitive Cube - the Blazing Moths in the garden (infinite but slower number of appearances)
West Ice Field - the Roball racetrack (dependent on the moving base's appearance)