Play the Evacuation level in Co-op mode. When you go outside, kill the Titan 
and all
enemies. Go to the platform that takes you to the next part of the level. 
Once you have
activated the switch, wait until you get high enough to jump down and kill 
yourself.
Jump below that platform. The platform will come down and squeeze you 
through
the ground. You will fall until you have respawned back in 30 seconds.