L.A. Noire

If you want this achievement fast, you can select the "Cases" option in the main menu (scroll down on when the screen loads - by default you'll get the "Resume" option instead), and replay any case with a large amount of enemies.

I went with the easy Patrol Desk case called "Armed and Dangerous". It's the second case in the file. Once the level loads, drive to the location, and you'll automatically retrieve a shotgun from the trunk. The shotgun has a large spread, and at times, even without aiming for the head, you may get a headshot credited to your kill, as shown in my video below:


You can check how many headshots you've done by pausing the game with , and choosing the "Statistics" option. Scroll down to the part that says "Gunfight" to see your total number of kills, and total number of headshot kills.

If you want to be sure of getting a headshot, press forward to the cars near the bandits. They'll retreat into the building, and at least one will take cover at a ledge near the windows. As his back is facing you, you should notice the head popping out (thanks, hat!) for an easy kill. Use the statistics page to check whether the kills count if required.

To continue the grind, let the last enemy kill you, and you'll restart right at the shootout. If you want to take a break, finish the level by killing the last enemy to make the game saves. If you don't save the game, all your progress will be lost. So either get the required number of kills in one sitting, or finish the level to save your progress.

You can obviously use the same method to get 100 total kills for: