Splatter!
This guide will tell you how to complete this achievement in the fastest way possible without cheats/mods.
Before you get started, I recommend combining the rock with down on the d-pad (

) and the axe/any weapon you choose with right on the d-pad (

). Also make sure you have a rock in your inventory too, These are found everywhere in the forest.
Assuming you know the basics of the game and have a shelter, Go out hunting for a small group of cannibals. Once you have found some or a single cannibal, kill all of them except from one. Once you have this last enemy alive, Continuously hit it until it falls down to the ground. Once it's done switch to the rock and hit one last blow. Sometimes the cannibals get up before you hit them, so this could take numerous times. Thanks for reading, if you have any problems or faster solutions feel free to comment below!