Achieve 1000 headshot kills. The quickest way to get this achievement, is to set up a match with the following settings:
Map: Old Town
Map Size: Small
Bots: 15 (Agent)
Round Limit: 10 minutes
You need to be a human, so if you spawn as the infected, just play the round properly. When you're a human and the purchase screen comes up, buy armor and FAL X2. Once the round begins, head up the stairs, near the truck in the middle of the map. At the top, enter the doorway to your left, then you will be in a room with three doors, a table and a doorway. Enter the doorway and run all the way to the end (over a little hole). Watch the doorway. As more enemies get infected, they will turn their attention to you. The bots will rarely shoot at you. They will simply keep running straight at you. Try to shoot 10 of them with your left pistol first, then alternate, left and right. Do not manually reload. Aim for their heads obviously. You can get 300-400 headshots every 10 minutes, so you're looking at around half an hour for the 1000 headshots. It can be a mix of online and offline. Teamkills count.
*You must complete the match for your achievement progress to register.