Gears of War 3 - Blackout

This chapter starts immediately. You'll be told to take an elevator at the back of the room. Before you do that, look at the elevator from the big room you started in. Turn right and there should be a giant closed door. On the right side of the door (behind some boxes) should be switch to open it and when you go through the door you'll be greeted by a Silverback. Take it to the elevator.
You'll manage to go up one floor... and get stuck. Follow hallway #3 first and you'll see barred doors on the left. One of them will be open and have a crimson omen on the wall. Next to the omen is a giant red door. Bust it down with the silverback and on the right side of this room are some COG Tags. This is also a good ammo cache so stock up on weapons (boomshot?). Follow hallway #2. You'll come across wretches and some flamethrower grubs. Destroy them on your way. You'll eventually come to a control room of some kind and in the back of the room you'll see the ladders to upstairs.

You can stay inside and flank right but it only helps you with a single Reaver. Do it anyway, then go outside and fight the Guards. After you kill a few of them, another Reaver should appear. He'll land basically right on top of you. He's kind of a tough bugger for some reason. It just seems the space was made for him. But if you head forward a little more, you'll find grenades on the ground which you can use to great effect against him.

Once that's dead, head down the stairs, turn left and left again, then go up a different set of stairs. You can see the control tower in the distance.
You'll also get attacked by a swarm of shriekers, some therons and an Armored Kantus. If you have no grenades, some spawn just before the last set of stairs on the right side. Take out the shriekers first because they'll come to you, clean up any theron guards that have moved forward and then go for the Kantus. Hopefully you can take him out quickly and possible finish the guards with a grenade aimed at the Kantus.

Once they're all done, head through the single open door and hit the switch to go down. When you get to the bottom, two guards will be waiting for you. Dispatch them and a flamethrower will come out from the right side. Once clear, go to the first desk and around the back side, on the left is a button that opens a safe on the dead end side of the room. Inside is a collectable. Go through the open door.

After the cutscene, continue down the next hallway. The side rooms contain some ammo but nothing more. However, they can be used to great effect for flanking if used at the right times. Watch out for Scorchers and follow the hall around the bend and downstairs.

Once you get downstairs, there are three switches to hit. The first is immediately to your left. Go around the generator on the right side and you will see the second switch in the same position on the right side of generator two. The last switch is on the left tip of generator three. This will bring you to a new control room and another cutscene. After that, you'll be deposited in a tunnel.

There's nothing in the tunnel, but at the end you will find what you are looking for. The first valve is on the central unit directly in front of you. It's not a normal switch but a strange square switch. There is another in the same spot on the opposite side of the unit. That's two switches down. In the meantime, be careful with the Theron running around and shooting at you from the side windows.

More grubs will appear in the windows as you go down the stairs at the end. When you get to the next level down, you'll find an Armored Kantus who is alone aside from the grubs in the windows. Take him out and flip the only switch on this floor, then proceed down to reach the last two. However, on the bottom floor there are two Armored Kantuses and they are pretty close to each other. If you're fast though, you can catch the first one apart from the second one and take him down before they can cause too much of a problem. If you need grenades, there are some just as you come down the stairs on the left side. Once you flip the second switch, run down the stairs right next to it and out the door. The game will constantly give you new waypoints to lead you out. You have 60 seconds to do so and just before the end there's a grinder. You should have time to take him out if you hurry, otherwise just run strait through. To the back right corner of the room.