Description: Headshot 3 enemies with the Assault Rifle and 1 enemy with the Pistol.
Note: All perk-based achievements can be obtained by re-loading the checkpoint and keeping progress. See video in description for details.
Doable early on, but if you need a more controlled environment try sneaking through mission to line up the shot.
You can also get a headshot, reload the checkpoint and you're progress will be saved.