To get this achievement, you'll need to be in the Dark Zone, and have to LAND THE KILLING SHOT on a yellow (elite) enemy that has a name. I have found two named yellow characters in the first area DZ01, the first one likes to hang out by the blockade (north) with his other yellow buddies. The second (which is much easier to land the killing shot because most people don't go down there) is in the Subway near the Greeley Square underground entrance. Be aware you need a Level four filter to go down in the subway. Both of these elites are level 12, and I suggest being around that level with decent firepower or they'll wreck you if you are not careful.
As a side note, I was in a team with someone and we took the subway guy down together, he landed the killing shot, I did not get the achievement until I came back later and killed him by myself.
EDIT: If you go above the level 15, the first area will scale with your level every so often and the yellows will be higher level as well.