The Last Stand: Aftermath (UP)

Once you get to the last region called "hell" you will encounter some new enemy types. One of these new enemies is called a flamer. He is a black skeleton that's on fire. He is usually surrounded by 3 other black skeletons that will chase you if they spot you. Kill these normal black skeletons first and then focus on the flamer skeleton. Keep a distance and just shoot him from far away as he spews fire when he takes damage. Once he dies he spews fire in an area around his body but nonetheless, you will unlock the achievement as soon as he's dead.