"This is a secret achievement which is unlocked by going to the second floor of the building in Chapter 6, Clean Hit, the player can see a thug sitting on a toilet. On the same floor a fireaxe can be found hidden in a container on the wall inside the main hallway. In order for this achievement to be unlocked, the player must kill the guard that is sitting on the toilet with that fireaxe. You will get the achievement once you enter the car. "
That's all the info from the wikia.com. It's as straightforward as it seems