Before you go, take some items: the Ceremonial Dagger, the Lair Map and the First Aid Kit.
Save the game (you will be able to get back here to do it as often as you want).
You don't have the rest of your equipment so you will need to fight with your dagger.
Go north, to the Room 212. Kill the cultist and take the Steel Pipe from the body.
Break the glass and enter the Room 211.
Dagger is still a better weapon than the pipe so use it to kill the next cultist. Take the Sector 3 Key and MK 23 Handgun. There is also some Shotgun ammo there. Look at the trashcan - you'll find Child's Drawing #18.
Xbox 360: collecting all 18 drawings will give you Nursery Rhymes achievement (50 p.).
Go back to the corridor - two other cultists will attack you. Shoot both of them or kill at least one with your knife.
Go to the place marked as Sector 3.
There is another cultist inside - stab him with your dagger. Go to the south-east part of this area - you will have to pry the door open with the pipe. There are two cultists in the corridor - kill them and walk to the Room 320.
Jump over the fence on the left, move the blockade and pry open the door.
He has a very dangerous weapon but in fact he is very incompetent. You can stab him with you knife without dodging, hitting him constantly. You can also kill him the usual way - dodging and attacking.
After the fight and a few cutscenes, go back to the Room 320 to take some things. Take the First Aid Kit and Health Drink from the table. Take the Room 301 Key from the wall in the Room 321.
Return to Elle and open the gate using the same method as you did in the sewers.
There is a pair of cultist in the Sector 3. Kill them quickly - one of them has a gun so he's twice as dangerous. Go to Room 301 (use the Room 301 Key).
This are many treasures in this room, but the most important thing is that you can get your whole inventory back.
Take Pistol Ammunition from the corner on the right and from the cabinet. Take your equipment from the table and Health Drink from the cabinet in the corridor. There is also a briefcase with Serum #8.
Xbox 360: collecting all serums will give you the Health Junkie achievement (50 p.).
Break the glass and enter Room 302, take the First Aid Kit and return to the beginning to save the game.
Walk to the Sector 1 area.
Go north from the save point and enter the door, then turn right and go east. Open the gate with a help of Elle. Enter the Room 113.
Take the Police Marksman Rifle and Shotgun Ammunition.
Break the glass and enter the Room 112.
Wheeler is dying but you have a choice (read the Ending chapter of this guide) to help him or not.
Xbox 360: helping Wheeler will give you Now about those drinks achievements (20 p. ).
Go to Room 111 and to the corridor. Go east, save the game and enter the room.
You will notice a riddle on the floor.
Use the control panel and press the X ( or right mouse button) button 2 times and A button (or left mouse button) 8 times.
The second part of the puzzle:
Examine the door and turn the outer circle 3 times clockwise. Turn the inner circle 4 times counter clockwise.
Go back to the save point, save the game and enter the crypt. Examine the sarcophagus in the southern part of the room.
This is the final boss and although he's not very strong, he's resistant and tough. If you saved a few medkits for this fight, you don't have to worry about the result.
Choose the axe as your weapon and run around. Attack him when he's vomiting. After a while he should transform into his second form. Use everything you have, every weapon, every Health Drink. Attack as hard as possible and after a while you should be able to perform a finishing move.