From Zelda Dungeon Wiki
The Minish Cap
Rock ChuChus are found mainly on Mt. Crenel and Veil Falls. To defeat them, Link must break their helmets by either throwing a bomb on it or by breaking it with his sword once he has gained the Rock Breaker ability.
|Appears in various areas. These tough Chuchus have rocky heads. It will be hard to damage them until you can knock that rock off.|