Pistons do not deal direct damage. You must exploit environmental damage (suffocation, crushing, or void drop) with the piston as the final "weapon."

: Maximize the hearts of a Creeper until it explodes.

The moment the mob dies, a toast notification appears: “Piston Trap Master” (or similar). If you see loot pop out but no achievement, the kill was caused by fall damage or a previous injury. Reset and try again with a full-health mob.

: You can change the appearance of the piston by purchasing Black Ink (for a black piston) or Scissors from the farmer to unlock a "ball-less" variant. LCPT 0.2.999 is released! - Lovely Craft by Crime