If you're having trouble doing this legitimately it can easily be obtained by using hollow-point bullets. Hollow-Point Bullets is a tier 2 Survivor skill and crafting one requires a regular pistol bullet and three magnesite ore. This makes the challenge easy as hollow-point bullets aren't part of regular ammo pool and don't require a reload so you don't need to worry about missing a shot or aiming for headshots. If you're struggling or want to give yourself further room for error then also equip the Siberian Ranger outfit which gives increased special ammo capacity. This allows you to carry upto 70 hollow-point bullets at which point even the most heavily armoured enemies won't hinder the achievement.