Level 10: Control
Get out of the building
Predictably enough, the whole Jack-waits-around-in-the-ventilation-shaft plot doesnt quite satisfy the exigencies of compelling storytelling. Mosey along the sewers until you reach another interior level. You should hear the sounds of gunfire coming from beyond the first door here; this is the result of a firefight between a few scientists and a group of Tridgens. If you dont want to get caught in the crossfire, you can either wait them out, or shoot the grate off of the nearby vent and crawl through to reach the scientists. Youll have to kill whoever survives the battle either way, of course.
When youre done scrounging for items (and avoiding the insta-death hissing steam), climb down the ladder and start crawling through the ventilation shaft. Rotate the steam valve at the end, climb back up and proceed down the now de-insta-deathed hallway. Shoot the scientists, and watch out for Tridgens in the venting in the nearby corridor. There are actually two vents in this corridor; first, follow the one that the Tridgens were crawling through. This will take you back to the steam valve room, so climb up the far side of the chasm until you reach another valve. Spin it, then proceed back to the corridor and into the other ventilation shaft.
Youll eventually come out into a room populated by mercs. Dispatch them, steal their stuff, and take the nearby lift. After killing everyone in the subsequent room, head out through the eastern exit and take down the Tridgens. The next room you pop out into contains three or four of the security-team mercs, they of the helmets and extremely dense body armor. These fellows are tough to kill, so whip out one of your assault rifles and try to pull off a few headshots apiece. Hopefully youll be far enough away for them to stutter in their return fire; if they mount a serious defense, just throw down a few grenades in their direction and back off.
The other exit from the room at the bottom of the lift will lead you to another medical research facility, where youll get your first glance at the massive Fat Boy variant of the Tridgen. Its dead, thankfully, but take note of the rocket launcher on its arm: youre going to be seeing one of these in action soon enough. The east exit of this room leads you back into the room where the soldiers were; if you popped a few of them, drop in to grab their armor and a medkit. The other exit from this room leads to a little shooting gallery of Tridgens; kill off the mercs, grab the minigun, and go to town. Eventually, a Fat Boy will appear, draped in shadow, and firing off his slow-moving rockets at you. Hes not incredibly accurate at most difficulty levels, so if you can, keep firing away while he attempts to move towards you. Even a Fat Boy cant stand up to many rounds from a minigun.
As you head down the hallway, youre likely going to attract the attention of the mercs in the kitchen off to one side; they seem to pick up any sound that you make, so a fight with these guys is unavoidable. Whats worse is that they all pack Jackhammers, making the close-range fight quite punishing for yourself. If you have any left, chuck in a flashbang grenade before entering the kitchen, then eliminate them as they flail about helplessly. Proceed from here through the head, offing Tridgens as you go.
Youll eventually reach what appears to be a reactor room, the first in a series. Youll have two exits, again; one in the floor, underneath a pool of water, and one in the wall. Take the one in the wall and climb the ladder to the top. This should start another smorgasbord of destruction in the room below, but luckily, none of the participants will be able to see you or shoot at you, providing you stay back from the edge of the pit in the middle of the floor. After the sounds of gunshots fade away, you can start taking potshots at the Tridgen and the Fat Boy below you; the Fat Boy will obviously require the most damage to kill, so dont hesitate to unload a few rocket rounds on him. Even if it doesnt seem like hes getting hurt, he is, so keep firing away until he finally keels over. When hes gone, climb down to the lowest level of the room and sneak out through the door nearby.