VS. Mode: Kill 10 players with headshots on a single match in a public match
For a kill to count as a headshot, the enemy's head must pop off. You will also see an icon with a head inside a circle beside your name after each "headshot", so just look for this icon each time. Headshots with Assualt Rifles do not take the enemies head off. The Pump Action Shotgun, Desert Eagle, or either sniper work well for this achievement. Also, I found Warzone to be the easiest to get 10 headshots; you have more time and people are focused on the objectives.