To unlock this achievement you need to successfully shoot someone in the head. If they don't die instantly, you have not hit them in the head, so you need to try again. A sniper rifle would be the weapon of choice for this achievement, but any gun will suffice.
To use the scope on a sniper rifle, you must first have it equipped, then when you press once, you will activate the scope, then to zoom in and out press and respectively. When you are looking through the scope, pressing from now on will fire the weapon. To cancel the scope, press .