You must hold your breath will shooting for the headshots to qualify. The sniper headshots can be farmed at the start of the last level, "Absolution". The level begins with a sniper rifle and 10 bullets available. There are more than ten enemies that can be found in this area. Kill as many as possible with headshots, and restart from the checkpoint when are out of ammunition. Your overall headshot kills will be retained and you will have ammo again. Repeat the process as many times as needed.