You may end up getting this through natural progression as you play through the game, but if you're like me you may still have some headshots to get after finishing the story. The easiest way to rack up a bunch of headshots is to play a Survival mode and select "Backyard" level.
On the first wave, there will be a bunch of the enemies wearing orange outfits charging at you. These enemies have pretty big heads, so it's easy to line up headshots on them. Simply switch over to the pistol and get as many headshots as you can. After wave 1, just let yourself die and press RT to restart the game. There is basically no load screen when loading up a new attempt at the survival mode, so this is a really quick and efficient method to rack up headshots.