Mendo's ring, hold in front of the moon in the windw of the third floor center room.
Anyways, easiest way (meaning, little or no training) would be to go with mana bombing or avenger.
For mana bomb route:
Levels 1-9: Increase Magic Attack, spend skill points in basic attacks, but do get charge and heal as well.
Levels 10+: All attribute points into more mana, all skill points into mana bomb.
Strategy, bomb them off the start (unless they have a really weak shield, in which case, you attack them with other magic attacks first, then bomb post-shield), use charge/potionn to get mana back, bomb, rinse and repeat.
Same goes for avenger, all strength and attacks till level 9, then all HP and avenger after.