My first guide.
This is a fairly simple achievement to obtain.
-Go to Matchmaking
-Select the Anniversary Playlist (from here you'll have several options. I picked Anniversary Squad).
-Once in a game, you simply need three head shots in a row. It does NOT have to be a triple kill, nor does it have to be with the pistol. I got mine with the DMR. Simply make sure that your three kills are all head shots, without dying, and this cheevo will pop.
Note: It MUST be an Anniversary playlist. It WON'T work in other gametypes, like Team SWAT for instance.