Headshots can be performed by shooting in the head of a monster. Everyone on the earth has to reach together the amount of 1 000 000.
If you're using the new TA App and want to unlock achievements in that simultaneously, have this solution pinned while you complete the challenge and you'll unlock:
As mcnichoj says :
During chapter 12 there is a good segment where a bunch of enemies spawn.
The sniper should be able to take out the head with one shot on casual or two/three with pistol/shotgun unless you have them upgraded.