Reach the part where you face the first normal Wraith. After killing it and the Prowlers, kill the turret with a Ghost and use the Ghost to ramp off the place where the turret used to be, onto the start of the "tower". It is easy to find, but tough to do. After you get up there, you will find a place where you need to grenade or partner jump. Do so until you cannot get any farther up the tower., then fall off to your right (you may want to only have one person do this). You should fall, then float in mid-air. Float to the rocks or mountain beside you. You will now be out of the level's boundary.