This achievement can be done even if you die and load a save. It's recommend you save before going into action, if you die, load the previous save. Works on easy difficulty too but keep in mind that the NPC's die if you hit them anywhere else but a headshot since it's on easy, so you might have a few NPC's that don't count. Very, very easy achievement that should only take max 10 minutes, but likely will require a reload if you die. Can be done immediately in the first level, highly recommend you take mp40 and the two pistols with you before you start the mission to make this achievement easier to do.