Go forth, using the only way available. You'll see a panel that will enable you to turn on the wall walk. Use it and go along the path to reach another room. The way leads through the hatch on the wall to the left. You'll see a portal, but before you use it, load up your energy gun.
You'll find yourself in a room protected by some hunters. Start exterminating the enemies, watching out for their reinforcements. When you defeat them all, jump down and get rid of a few overgrown dogs. Gather some ammo and find a switch that will turn off the alarm. There's a door nearby, use it.
Leave Tommy's body and get through the force field. Push the button labeled Floor 1 and wait for the platform to come. Go back to Tommy's body and enter the bridge. Once again spirit walk and use the panel. This time press the button Floor 2.
When the platform that Tommy's on will go up, head to the door. Go along the dark corridor, dealing with the enemies. After a while you'll reach a room with motion detectors. Spirit walk to find a button that will deactivate them. When you're back to your body, gather the ammo and continue on.
Time to battle your new enemies, spider-like creatures. It won't be easy - they can disappear and reappear at random, so you have to be focused. Grenades won't do the thing here. The monsters create yellow acid clouds, that stop them from reaching them. You have to use a machine gun and an energy gun here (the terminal to load up the energy gun is near the wall walk).
When you defeat one of the monsters, pick up its rocket launcher - it's the ultimate weapon in your collection. When handling it, you'll be able to create such bullet-stopping clouds as you've just seen. It will surely save your butt a few times.
When you destroy all the enemies, use the wall walk to get to the higher platform. Recharge your energy and head to the door that led you to this room. Near it, there's another crate. Use it to get to a small room, where you'll find some additional ammo.
Going forwards (past the gate - #273) you'll reach a corridor, where a concrete block will cut off your way (#274). Find a grav-switch on the ceiling (#275) and shoot it. Go forth and shoot another one. Open the door using the alien's hand (#276).
You'll reach a room filled with strange machinery (#277). Go forth and wait until the moving element is fully on the left side (#278). Quickly run to the other side and approach the window. The glass will be broken by the enemy in the control room (#279). Kill him and enter the room. Press the button here (#280) and leave.
Defeat the flying creatures and wait for the machinery to move to the left (#281). Get across to the other platform and run forth. At the end, where you'll be attacked by the hunters, you'll see a button (#282). Press it.
The button turned off the blue ray that would instantly kill you when touched. Wait for the machinery to move to the right and then enter the small platform attached to the construction. Let it take you to the other side (#283).
Wait for the blue ray (#284) to disappear and then enter the pipe to the left and run. Half-way through you'll find the exit (#285). You'll have to do it quickly - if you take too long and machinery comes back to its original position, you'll be fried before you know it.
Go forth. When you reach the railings, spirit walk and go across using the glowing bridge (#286). Enter the button (#287) to move the pipes. Go back to your body and use the passage you've just created.
After going through the door, you have to deal with another group of monsters. Kill all the enemies and go down. Reach the opposite end of the room and jump from platform to platform (#288) to get to the higher level. The next gate will be covered by green ooze (#289). In order to get rid of it, you have to push a cocoon near it and then shoot it.
Go forth. After a short walk you'll see a blue portal (#290) that will let you finish this stage.