Level 7: Bunker
Get to the helicopter landing pad
Take the lift down to the interior of the bunker and kill off the guards just inside the first room. There should be four of them all told, two of whom will be wielding Jackhammer shotguns. When youre inside interior levels like this, you can feel free to get rid of your sniper rifle; just drop it, pick up the Jackhammer, and walk over the sniper rifle again to get the ammo that you left in the weapon.
Your path from this room leads either up or down a flight of stairs; taking the high road is a bit easier, as it lets you pelt the scientists in the next room with grenades from around the corner, then shoot them from the catwalks. You can shoot through the grating, so feel free to aim at people directly below you. When youve cleared the room, check the path from the lower flight of stairs to find some more shotgun ammo and grenades.
Again, there are two sets of exits from the operating room here, one high, one low; both will similarly wind up in the same room deeper in the bowels of the bunker. Kill the soldiers at the foot of the stairs, preferably by just chucking a grenade through the slots; this will ignite the oxygen canisters and create a sizable explosion.
From there, youll come across yet another choice of paths, but these actually lead to different locations. Of the two doors leading to the water pit, take the rightmost first; kill all of the soldiers nearby, then walk around to the security office to find a sizable ammo cache, and more ammo and health. From there, head back to the left door and proceed on your way.
The next few rooms are linear and without surprises, unless you insist on flipping the switch next to the locked Tridgen cages. Youll eventually have to pass back through the water pit, where a few more mercs will be waiting; if you dash out to the minigun and paste them, you should be able to escape with minor damage. If you do get shot up, return to the last room that overlooked the water pit and get some more health/armor, if you didnt pick them up earlier.
The next major fight occurs around a stairwell, with two mercs taking on four Tridgens. As you might surmise, the mercs dont fare too well, leaving you with a few bloodthirsty mutant primates on your hands. Chuck a few grenades down the stairwell as the mercs attempt to fend off the Tridgens; this will hopefully even up the odds a bit, letting you lure the Tridgens back to the doorway and pick them off with your Jackhammer. From there, its a fairly straight shot to the Tridgen holding cells.
When you reach the cells, sprint up the stairs to your left to find the rather user-friendly switch to open the Tridgen cages. The ensuing carnage should Darwinize the weak elements in the room below; use your M4 to take out any remaining Tridgens. The doorway below leads into a kind of loading dock; walk in far enough to trigger the arrival of the Tridgens from above, then retreat and let the mercs and mutants sort out their differences in an amicable fashion. When you dont hear any more gunshots, proceed on through and finish off the remainders. From there, its a short trip to the helicopter pad. Thrilling cutscene, kick in the face, and youre moving on to the next level.