|Steal||Potion, Wrist, Mage Masher|
As the first boss of the game, Masked Man shouldn't be terribly difficult. In fact, some of his attacks will fail where he'll trip over his own feet and fall flat on his face. Four members of the Tantalus group are at your disposal, so have all four attack until he is down.
Alternatively, you may wish to steal from him as he holds a Potion, Wrist, and Mage Masher respectively, which can come quite in handy.
After the battle you will gain a useful tip, which is to make sure that you use Potions to restore lost HP!