This one’s the hardest “weapon” achievement. The pistol takes anywhere from1 -3 shots depending on where you hit them. For easy one shot kills, approach the enemy then back away. The enemy will run at you, once he gets in the running stance, shoot him in the head. Shiloh is a very good mission for this achievement, you will be able to get most, if not all the kills you need on this level. Must be done with the Colt and/or Remington. This achievement is glitched, go for it once you start playing or it may not unlock.