THE HOUSE OF THE DEAD: Remake

This is cumulative over your entire time with this game, any mode, win or lose. You will almost assuredly unlock this naturally well before finishing everything else. Personally, I actually unlocked this while using the grenade launcher to just plow through chapters, so you actually don't even need to worry about using a gun and aiming for the head necessarily. If you somehow find yourself still in need of this achievement though, just replay chapter 1 and use the starting gun to make it easiest to just aim for the head.