Have you done it and killed the monster?? You have to kill the monster before you kill the man! It’s pretty hard

This is what I read on the internet

How to change the costumes:
When you are in the ice cavern and you beat the giant monster that the little man summons without using magic the man will give up and he will give you an item and run away. Now go to deli village and in the hotel you can get changed in to a different costume.

why would they lie about a thing like that? I think if a man gives you an ITEM then the chances are more likely.