Note: This game is also titled Dragon Quest Sword: Kamen No Joou To Kagami No Tou.
The Rock Golem will hit you multiple times before you can hit him. Use your shield to block them, then hit him when he is stunned. When he is at about one quarter health, use your mega strike to almost defeat him, then strike him down. Remember that the prince can heal you if you run out of medicinal herbs. It does not matter which way you go to get to him.