I did the same thing you mentioned to boost their MN when I was using the XP eggs, but that was a tedious enough job in itself.
The thing is, you could do without all the leveling if you ignored the ArenA. You don't really need them leveled so high for the rest of the game. But who wants to skip the ArenA??????

In LFIII I did all my leveling at the trial of Kondler. These blue-green snake-like thingies that respawn give a good amount of money and I became rich there!