Gamer has been removed
One of my favorite easy achievements!
Create a private LIVE match. Co-op defending base, auto-starting at 30 seconds between matches, and just let the game play while you are sleeping or at work.
Just make sure you have a wired controller so it won't shut off and change the setting on your xbox if it powers off after too much idle time.
*EDIT* I forgot to mention (and it was brought to my attention) to make the game length as short as possible. The shorter the game the less minutes needed!