One of the better levels to acquire this achievement is the mission Departure which takes place on a collection of Rooftops with brief interior sections. It’s also the first place you can acquire the Dragounov. The rifle is an automatic pickup following a cutscene, after which you are tasked with taking out several enemy snipers and foot soldiers. Several of the enemies on the rooftop closest to you will drop the Magnum Revolver. I suggest swapping your 9mm Pistol for this beastly weapon; with a larger caliber and clean iron sights, this gun is a headshot machine. Take your time, line up your shots, and use these powerful options to take out 15 foes! You have an AI companion who runs interference, but stays relatively passive, causing the enemies to stand still and make easy targets of themselves. These kills don’t have to be sequential, and should come far before the end of the mission. Hope this helps and feel free to let me know how I can improve this solution!