OBJECTIVE: Leave this place
The only enemies on this level will be the demons flying out from different places. First, gather the equipment from the room at which you've arrived. On the desk there's a Medkit and an AT-14 Pistol, next to it you'll see an MP-50 Repeating Cannon. On the boxes there's a Body Armor and a VK-12 Combat Shotgun.
Go outside the room. On the left there's a Medkit. Go straight on, then left as it's impossible to choose the way on the right due to the explosion. There'll be another explosion - jump over the rail on the left (there's a 10mm HV Penetrator here) and go up the short stairs.
Be careful - you'll meet the first wave of demons in front of you. Kill them and go in the opposite direction. Under the window on the left you'll find a Medkit and a VK-12 Combat Shotgun. Save your game.
It's going to be quite tough in here, because the demons will come flying from three different directions. It's highly unlikely that you'll be able to defend yourself on the one side, so at least get rid of the demons on the two remaining sides. Go left, you'll find a Medkit - next, go by the wall perpendicular to the windows, and then go right. On the floor there's a Health Booster.
Go backwards to the next room. Soon, the demons occupying the previous room will fly in here. Kill them all, then take the Medkit and a Body Armor which are on the box.
On the left, against the wall, there are two RPL Sub-Machineguns. Keep going further to the double door and then enter another storehouse.
You'll encounter another wave of demons. Go left and up the stairs.
Walk to the end of the bridge, there's a Body Armor there. Next, save the game and jump out. A difficult battle awaits you - there is a whole lot of demons. Use Slo-Mo - you can turn it on now. The RPL gun is also helpful here.
After the fight, take the Body Armor and an MP-50 Repeating Cannon which are on the right. On the left there's a Medkit and an RPL Sub-Machinegun. Approach the wall on the right, you'll find another Medkit on the wall, and next to it there's another RPL Sub-Machinegun.
Now, go to the other side of the room. Near the barred double door you'll find a Medkit and a Body Armor. Jump into the pipe and keep going in the tunnel. You'll beat two demons inside.
Keep going further, one the left you'll find a Medkit on the box. In the corridor you'll encounter another wave of demons - this will be the last one. Follow the only available way.
Go straight on in the long corridor. The lights will start to go down one after another, and you'll be transported to the world of visions. Open the double door, you'll be crossing the burning room once more. There might be some demons in here. Go left, upstairs, and open the double door.
Keep going straight in the corridor, next, turn around and start shooting at Alma. When you return to the real world again, keep walking down the only accessible path till you get near the elevator. Go upstairs - once, you're there jump out of the window. The game will end.
PS. Watch all the credits to hear an interesting conversation.