Go to OPTIONS and then select PASSWORD
Then enter the code YWW4 7HVD BCLG RKVB
and follow this very brief video.
Edit: Saw3d 0ff has provided a text solution in the comments as well. Here it is:
Easy achievement, takes 1 minute or less to unlock.
For those who can't load the video:
1. Enter the password in the solution
2. RB until you are wielding the Unmaker (it shoots a lot of red lasers, and is the last weapon in your rotation)
3. Hit the switch in the center of the room, and go down the chute to the right. The demon is to your left and straight away. Keep firing and strafing left and right. Easy.