Piercing Blow is a badge that uses 2 FP and deals damage that pierces enemy defenses. It can be purchased at the Lovely Howz of Badges for 75 coins. It uses 1 BP.
This badge will only be useful against enemies with an abundant amount of defense (e.g. at least two points) as it deals with the base amount of damage your hammer normally inflicts.
Note: It does NOT work on the Iron Clefts fought in the Glitz Pit.