Fafhrdd263,869 Locked 25 Mar 2009 21 Jan 2011
To get this you must play a co-op game and get to and beat level 20. You must play co-op over Xbox Live because you can't unlock this by playing local co-op (Thanks Robotic Aquatic!).
It can be accomplished with 2 players however playing with 3 is recommended and playing with 4 makes this achievement much easier.
Remember in the first level you can skip to level 4 or level 8 if you use the exits on the right and not the one on the left.
You can use the level glitch if needed. I would highly recommend using it in level 10 and 14.
Level Glitch:
This is not really a glitch, I think the designers made it so if you did get stuck you could exit anyplace. It you stand still for about 3 minutes ALL the walls turn into exits. Only 1 person needs to stand still for 3 minutes, others can move and shoot and it would still work (Thanks Drakenden!).
Everyone in the group must have over 200 health for this to work (since you would die other wise). Just stand still and NO DOT hit the left stick (you can shoot all you want). After 3 minutes the walls turn into exits.
I recommend standing near the start of the level.