1. Go through that doorway on the right.
2. This rubble comes crashing down from above. Don't bother with the battle through the opening on the right. Instead, head left. You'll hear strange noises and then this explosion opens the way ahead.
3. Drop to the ledge along the right then climb the ladder to the opening seen here to the left of the ladder. Update: David Falkner sends this image which you'll find at the bottom of the shaft -- if you live. (You'll need a cheat code.)
4. A medium medkit and armor can be found along the right wall. Crawl alongside these pipes to the next room.
5. Shoot the explosive barrels at the bottom of this hole. Drop to the ledge jutting out then drop all the way down.
6. Take out this guy across the way. If you hadn't already shot out the explosive barrels, he could shoot them now, killing you.
7. Turn left, then continue through this door on the left after grabbing the large medkit and armor vest through the door on the right.
8. You can avoid a firefight through the right door and just go up the stairs.
9. Watch out for this guy hiding behind here.
10. Drop through this ladder on the left.
11. I found Adrenaline Mode to be useful for this firefight.
12. Climb through this hole for a very not-fun jumping puzzle.
13. You need to start out crawling, not jumping. Crawl left, then right, then circle around to this middle area, where you begin jumping.
14. Climb straight ahead and use these broken metal rods as a ladder to go higher.
15. Now you can drop straight ahead.
16. Lastly, crawl through to here, drop to the ledge and climb out.
17. This battle is another good time for Adrenaline Mode.
18. Here I am being killed. That door ahead is where you want to go, and there are a lot of enemies behind it. I ultimately found it helpful to use the sniper scope and slowly chip away at them one at a time.
19. Health, armor and ammo are back here. Don't bother going downstairs.
20. Instead, continue through here.
21. Climb down that ladder and hop over the bent wire mesh.
22. Battle your way up these stairs.
23. More big guys wait for you up top. If you head right you can ramp up some debris where you'll find a large medkit and an armor vest, then swing around back to this position. Continue through the door ahead.
24. Crawl into that open ventilation shaft to load the next level.