very easy - make sure you're at a location that has few obstacles, such as crates and such. I got the cheevo on the map that's a four-way road intersection. There was an enemy right in front of me, I shot him in the leg so he'd run. I then popped a Reactix pill which slows down time for 10secs. As he got back up, I zoomed in, and as he started to run I blew his brains out. Pop!