Wonder Boy in Monster Land

This is the boss of Round 6. He will ask you a question; in order to engage him in battle you must answer this question incorrectly. This is one is one of the toughest bosses in the game, so make sure that you stock up on magic on your way here. You can simply spam this, or you can defeat the Sphinx by jumping towards him and striking as soon as you deflect his lightning attack (you'll know when you've done this, as you'll hear it as it strikes your shield.)

If you want to make sure that you'll get the question wrong, make a save before entering his chambers and choose a different answer should you need to.