A really easy way to achieve this, is on the mission "The King of Chinatown". Once you open the gate and have to locate the King, run (or speed walk) all the way to the exit to the area on the far side.
When you get there, you will see 3 police officers stood together. Just hold RB and press X. You will enter Point Shooting. Hold LT to aim, and press RT to "tag" the heads of all 3 enemies. Now press X to execute the manoeuvre, and achievement unlocked!