From the Super Mario Wiki
The Big Bone-Fist is an enemy encountered in Wario World. It appears twice in Horror Manor, and as its name implies, it is a large skeletal hand attached to a wall that Wario must defeat in order to proceed in the level. The Big Bone-Fist attacks by grabbing Wario and throwing him, shooting energy rockets at him, and attempting to slam him. The first attack can be escaped by wiggling the Control Stick. The other two can simply be dodged.
To defeat the Big Bone-Fist, Wario must wait for it to do its slam attack. A weak spot glows blue on the back of the Big Bone-Fist's hand. A Ground Pound on the weak point damages the Big Bone-Fist, which needs two more Ground Pounds to finish off. As it takes damage, the Big Bone-Fist gets more aggressive and fires more energy rockets.
Oddly, despite the name, the Big-Bone Fist never makes its hand into a fist.
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