Sharpshooter
15G - Kill 500 infected with a critical shot.
A critical shot is a head shot which can be done by aiming at an enemy until the marker "scopes" in and turns red (or whatever color you have it set to in your game.)
If you are going to complete the game, you will have this done relatively fast. To speed it up even more though, you could drink "Cyan" potions to speed up your targeting time and critical hit damage.