Killing the last aliens, then jumping into the teleport to finish the level.
For whatever reason, there's no barriers of any sort.
The Marine begins at a drop-off down into a water area crawling with Advanced Usurpation Soldiers armed with Alien Shotguns. The first hostiles that are encountered are the Drones that will fire at the player as they descend into the water area. The Marine arrives in the water area and sustains heavy fire from Alien Shotguns. After taking out the Advanced Usurpations, the Marine crosses the water area on a platform beneath an even higher platform with more enemies that will fire down at the player. The Marine must proceed to make his way down into a pit below the main area of the level and teleport to the Usurpation Ship in the next level.