In this level, you will need to shut down the reactor and life support in order to provide enough power for the teleporter. The more difficult section of the level starts once you've shut down the reactor, and descend the elevator after the Z-Sec soldiers.
After starting the level, you begin at the skytram station. Enter the left skytram, and use the screen inside to activate it. When you reach the next building, you can disembark and pass through the door. After the door will only have zombies attack. If you enter the plasma storage to the left, you can find plasma cells and a health station, but killing the zombie inside will cause demons will teleport in: Two vulgars and one revenant.
When you first enter the core, a mix of cacodemons and lost souls will teleport in at the middle of the large room. Simply continue to the right and descend the lift. Additional cacodemons and lost souls will arrive from the middle. Descend again, and go through the door.
In Lower Maintenance, descend the ladders, killing any zombie that blocks the route. Get the PDA of Nicolas Haag and the access key. The PDA also lists the security locker code of 492. Collect any items and armor. Return to the top - you can take the ladders, or you can take the lift (which will also spawn two Vulgars.)
More cacodemons and lost souls ill spawn on the second tier of the reactor core, with much more lost souls then previous waves. On the upper floor is the final wave of one cacodemon and two lost souls. Enter the door locked by the access key.
In the control room, initiate shutdown. Open the locker (code 492), and return to the reactor core.
The control panel just outside the control room is now active, and you can use it to extend the bridge. Cross it, and enter the next room.
Reactor Support will initially have Z-Secs on patrol, who will attack when you enter. When you descend the lift, there will be five revenants that will approach from the corner, as well a pinky demon that will approach from the door.
Going through the door will bring you to Energy Processing, which is a dark area where monsters will arrive and attack. At least you will be escorted by a Sentry Bot which provides lighting for you, along with some firepower. The other end of the dark corridor leads to an elevator, which in turn leads outside.
In the outside section, head to the left, down the ladder, and into the next airlock. The only worthwhile item outside is security armor one ladder down, and there's barely enough time to collect it on the first excursion. Cycle the airlock.
On the other side of the airlock are two Z-Secs waiting for you. At least there is a health station to recover health, and you may be able to. Enter the elevator and go down.
The cargo tunnels are a bit dark, but you can open the shutters to bring in some light. Enemies include Hell Knights and revenants. After the hell knights, there's one pinky demon after the door, and one revenant after you reach the stair case. Ascend the ladder after oging up the stairs.
You should reach a control room with environmental controls. Initiate the shutdown. This will unlock the door to the skywalk, which leads to the level's exit.