The best place to grass grind is at an Iron Door, as your army will attack it, and the door will receive no damage. Iron doors can be found in nearly every dungeon, but the Castle of Justice is recommended. Oohoroc's Volcano spell and the Unique Halberd, Incensar, work the best for grass Grinding, as they deal direct damage to the grass. If done consistently, you earn roughly 1000-2000 EXP per hour for a unit the same level as the Bowmunk. Note that a unit with a lower level will earn more exp and in effect take less time to level up.