Metro Last Light

Ashes is the second chapter in the game. The only real trouble that you will have to deal with is fighting the Watchers while your Sniper friend covers you. Keep strafing around them and quickly kill each one. The area you fight in is rather open, so you can be as far away from them as possible, giving your sniper friend more opportunity to kill the mutants, rather than you. As long as you don't take a hit, the achievement will unlock.