This level you run backwards from the Marine mission, but don't go quite as far. When it starts you're in the elephant man's room. Climb up the wall and through the hole where the elevator goes (the lift will not take you up). Go through the artifact and out of it. Climb up the wall above the door and into the tunnel. Run through the tunnel and turn right at the fork. Take the catwalk all the way across. Follow the tunnel into the computer room and go through it. Follow the tunnel again and go across the walkway in the generator room. In the next tunnel there should be a grate on the ceiling. Break it and crawl inside. On the second grate going back down break it and jump in. Destroy the turret. Turn around from the turret and take the path to your right. Follow it and destroy the next two turrets. Turn left and follow the path to the next computer room. Jump down and to the right. Break the two turrets down there and go through the path. Follow the tunnel to the artifact and jump down. Go through the first on your right. It's a way down. Go to the end and climb up into the air duct on the ceiling. Go through to the first grate. Break it and drop down. Kill everyone. One that looks like a civilian is a company head. That's one. Climb back into the air duct and to the last grate. Destroy it and drop down to kill everyone. There is another company head in here. Once he's dead, the mission will end