Obtain a sniper rifle in a ranked match, then find a spot to set up your camping position, stay back and shoot everyone in the head. If you know it's hard to hit your opponent, don't try it. You'll just lose ammo and have a chance of your opponent spotting you.
Even easier done with a friend. Grab a sniper and let your friend walk to a fixed open position. Then shoot him in the head 100 times and you're done.
It's not a hard achievement, but fairly time comsuming.