Go down the stairs and then stock up on ammo by the bathroom (frags in the bathroom) before entering the restaurant proper. Inside the main dining room, you'll encounter a variety of Drones and Theron scattered around the room. Move around the right side and you can use this to flank the lower middle section. Cross the middle section and go up the stairs towards the kitchen. Stock up on ammo from the fallen enemies here. Try to walk out with a Torque bow if you can.
In the Kitchen, you'll find several Theron in here with Torque Bows and an Armored Kantus. The Armored Kantus is exactly why you have the Torque Bow. If you hit it once, you'll wound it. After that, he'll scream and be vulnerable. Normally you'd have to hit him in the mouth to kill him but if you hit him with another Torque Bow bolt, it should take him out. This is much preferable because sometimes you don't have a good angle on the mouth. Finish up him and his friends before getting to the real work.
On one side of the kitchen will be a small pantry with a dead body in it. Your [COG Tags] are in here next to the body. You'll see a freezer in front of the door with a Crimson Omen on it. Now start going around to the three different Gas Mains and open them up. You'll start flooding this area with gas but this doesn't prevent you from using explosives. As you open the gas mains, enemies will start to come out.
The first wave should be light and consists primarily of Drones. After the second main, you'll have at least one Armored Kantus, Ragers and Theron. The third wave after you flip the third main will consist of two Armored Kantus, a Theron, and Ragers. Try to Chainsaw the Ragers and Torque Bow the Armored Kantus. This will make for the easiest fight. Watch out for the Theron however because they will have chainsaws themselves.
When the room is clear, you'll immediately leave the room and find yourself on a zip line. While on the zip line, you'll need to shoot the gondolas off of the line. Aim for the shining part that's attached to the line itself. When you hit that spot right, the cars will fall right off. When you land on the far side, you'll have completed Aftermath!
