This guide contains spoilers for the game's story, do not read it until you get the Windbreaker [Story Related, cannot be missed] if you don't want spoilers.
*** Spoiler - click to reveal *** After Pastor Al dies he gives you the Windbraker, jump from a height of at least 1 block, and press 
/ Space to use the Windbraker, once you have your Windbraker you can hit and it will do a dive hurting enemies below. Find a slime (or any other enemy) and kill it using the dive attack to get this achievement.