Specifically, the achievement requires you to kill a mob by activating a piston that pushes a block into an arrow, which then flies into the mob. (Yes, the game tracks the kill credit. No, you can’t just punch it.)
This achievement is deceptively simple, but new players often struggle because they don't know where to find (wait—no, that’s for Sticky Pistons). For a regular piston, you don't need slime. Let's break down the exact recipe. lovely craft piston trap achievement guide