Exit the room. You need to turn right, but don't miss a box with stuff on the left. Combat music will start playing. Your brothers in arms are fighting aliens. You cannot save other humans, so don't even bother. Go down the stairs and straight ahead until another skirmish starts. And again, aliens will kill another marine and you won't be able to help.
Go up the stairs and through the door. When you come closer, aliens will assault. Don't shoot them when they are closer to soldiers unless you want to hurt your allies and fail the mission. When the fight is over, go back a few steps and move down the ladder. Move ahead until you see a marine slayed, again.
Go through the only door that can be opened and down the stairs. Turn right, then left and you will come to some sort of technical hall or whatever it is. Stand in the middle of the hall and look around. You need a corridor illuminated red. Go there, up the ladder and smash the grille.
You will come to a landing zone. Two marines will be running to a gate. Follow them and get inside. Use the elevator to go up, turn right and follow the corridor until you reach the door. A few aliens are waiting behind it, so stay frosty.
Inside the room you will find a minigun. Pick it up and complete one of the tasks. Use the huge dashboard to finish another task. Switch to a minigun and prepare to exit the room as a praetorian will attack you. There is only one weapon that can deal with it, so use it.
Go out the door and you will see one floor tile missing. Jump down. Another pack of aliens has spawned, so you will have tough time heading back. You need to reach the landing zone. If you forgot the route, go down the stairs and follow the corridor to a machinery hall. In the middle of it you will see a red corridor. Go there, up the ladder and to the landing zone. There another praetorian will attack you, so kill it with your minigun or just stay alive long enough for the dropship to land. Once the transport arrives, get inside to finish the level.