From the Super Mario Wiki
Barrel Busters are snowmen in barrels, with their heads sticking out of the top, which appear in the Nintendo GameCube game Wario World. They are only found in the fights with Angler Manglers and Terrible Portraits. The Busters are needed to defeat the Angler Manglers and Terrible Portraits, which is done by stunning the Barrel Buster, picking it up, and doing a Piledriver under the Angler's glowing orb or one of the Portrait's arms, which will shoot the snowman's head up, hitting the monster's weak point. In the Terrible Portrait encounters, the Barrel Buster is made of cloth as opposed to snow.
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