You can get this achievement when you encounter the first batch of enemies on Day 1 (level one) of the game.
Go down the street you first land on & a group of enemies will attack you. Simply attack with basic X and / or Y (Y is better for delimbing an enemy) attacks until you have decapitated one of the enemies. Simply press Y near one of these decapitated enemies. Achievement unlocked. Even if you haven't noticed that you've delimbed one of the enemies, the following text will come up on the screen when you have first delimbed an enemy in the game; "Injured enemies are crazed at the thought of death. Put them out of their misery with Y.".
TL;DR? Start a new game. You will find the first group of enemies a short distance down the road you first land on. Decapitate one of the enemies with basic Y and / or X attacks (Y is better for decapitating enemies). Press Y on an enemy you have delimbed. Achievement unlocked. Have a good one.