Follow the walkway on the outside of the Rig until you get inside the building. In this first room, you need to navigate individual platforms by vaulting over the pipes in between them. Go forward and right but before long, you'll be stopped by a falling platform. You'll have to go the long way around by going left and you'll be chased by Fallen the entire way. They will continue to spawn in the area below the platforms and then they will jump until you get to the lockers with the gas masks. For that reason, don't worry about killing the enemies unless they are directly in your way.
Once you have your mask, stock up on ammo from the nearby room and then head downstairs. You'll have to make your way through the bottom level under the platforms you were just on to get out. Try to avoid the Imulsion pools. Most of the trip should be pretty painless but when you start getting to the exit, you'll be ambushed by several Fallen. Get through them and then get up the stairs at the far side.
The next path is pretty linear. Follow it until Baird says "ok, masks off". At this point, find the exit but don't go through it. To the left of the exit will be a door with a Lambent hand print on the door frame. Go through that door and then take the first right into the sickbay. In the farthest corner of sickbay will be the [COG Tags] for this section but be careful because you'll be ambushed by more Fallen.
Now exit through the large door and when you get outside, you'll be attacked by Lambent Wretches. Shoot through them and then make your way to the flare in the middle of the platform. Just to the right of that flare will be plenty of ammo. Hit the flare, stock up and grab the Mulcher. Bring it up to the higher level and off to the right side. This side will give you the best height advantage.
This is when the fight begins. This first wave will start with a Reaver. Kill the riders first and then lay into the Reaver with your Markza. After the Reaver is gone, start shooting into the lower areas. You'll have to contend with Drones and Theron in this first wave. The second wave will bring various Boomers and Theron with it. As you work your way through this group, another Reaver will appear.
Killing this reaver will start you on the final wave. Fallen will drop in from your left so kill them first. A third Reaver will appear and Boomers will start coming for you. Keep your eyes open for all three of these things because they will all keep coming fast and heavy until you leave. You'll have to hold out until the chopper gets close enough. Eventually Baird will order everyone to the Raven. When he does, make a run for it. When you get there, you'll finish the chapter.
