Hell Base Chapter 4
Default Horde
Easy Difficulty
There are plenty of locations to gain this achievement / trophy, but the surefire area is Hell Base Chapter 4.
Right at the start you'll be prompted to take the Relic. Shortly after between 6-8 Snipers will start teleporting in from the Balconies. Simply step back and take your time with the shots or don't and just be quick on the trigger.
Video Provided by Iron Goat Gaming