This one is pretty easy but it does require some setup. This achievement is unlockable right at the start of the level DEPOT, but you will need a silenced gun, preferably with a scope but not necessary, to kill the guard.
There are numerous weapons to be found in the level FONTLINE which is not long before DEPOT. However if you want to be sure you can get a silenced gun, load up ARMORY, buy an AK and attach a silencer to it and play trough the next few levels.
Anyway, collect a silenced weapon one way or another and make your way to the DEPOT level. Right at the start your comrade will say something about the station being deserted and how unfortunate it would be if there are enemies. To everyone's surprise though, it is not and there will be a guard standing on the tracks. Your comrade will mention mention this to you as well.
Now your goals is to kill that guard ASAP, preferably by headshot. This can be tricky. However what makes this a lot easier is that you can just try again by reloading checkpoint. You should get the achievement mere seconds after killing the guard. If you didn't get it then reload and try again.
Tip: I used a machine gun with MGR to make sure he was dead.