Belly of The Beast: Evolution
Listen to the dialog, then take the left path. Kick out the door and pick up some ammo. Try to open the door at the end of the corridor. Not working? So you'll have to walk around to the right path. Open the working door there and continue moving, grab the ammo to the left. Reach the next door and kick it out. Proceed through the next door, meet the Stranded guy and watch the cinematic.
Now follow the Stranded and let him open the door for you. Continue following your guide while listening to the dialog. There will be a small room to your left with some ammo if you need it.
After the Stranded's unlucky demise, pause a bit. If you break through the planks and fall down, you will find yourself fighting an endless stream of Lambent Wretches, with the only exit option being a ladder in one of the corners of the room. The ladder leads you to the beginning of the unsafe planks area.
For the time being, turn right, walk past the machinery and immediately turn right. Walk to the end of the room - here is your twentieth COG Tag, reachable by walking directly on the supporting plank. Now turn around and walk near the wall to reach the exit.
In the next room, grab the ammo and the grenades you rightfully deserve, and exit through the door. After some more ammo and another door, move forward and prepare for some fighting with the Lambent Wretches. When playing co-op on Insane difficulty, we devised a daring, if extreme tactic: both players run back to the previous room, one gets in, the other one stands in the door and falls, blocking the path for Wretches. The guy who still stands first kills all of the monsters, then helps the sacrificial player to get up. That's Corpse Tanking (TM). Might be unnecessary on lower difficulty levels, though.
After the fight is over, get through the door at the end of the room and turn right. Run forwards, turn left and use the valve. Grab the ammo in the room to the right. You also have the opportunity to switch your revolver for a pistol, however, I strongly advocate against it. There's also a shotgun here - now that might be a worthy thing to pick up, depending on your preference.
Exit through the other door, get down and reach the ammo. Proceed to the next room, pick up more ammo if you're able and meet the other team members. Turn the valve to open the gates. Enter the gates and take on some more Lambent Wretches, as well as the drones.
Proceed through the tunnel where the drones came from, turn left and kick at the door. As the gate opens, fight off the Boomer and drone.
After the fight, the door will open. Proceed through, pick up everything you need and head down to the carts. COG Tag number 21 is at the left side of the stairs. Go to the cart control room to end the chapter.