At this point, you are trying to retrace your steps but stay under the lip of the road and out of sight because you have no weapons. Keep crawling along until you enter a sewer drain. Crawl through the pipe and kill the rat with the QTE. Climb out of the pipe and turn left. You'll be re-entering the building with all the rugs. Come up behind the guy ahead and kill him. Immediately after that, use his weapon to take out the two enemies who are now firing at you. Be careful of enemies entering the area from the left. Once this part is secure then you should proceed down the hallway to the right and at the end, exit the building to the left.
You'll have to run down a street headed back towards the garage where your squad mate was shot. This section is probably your first real challenge. Clear the area but take your time and stay prone. Make sure you watch to the right as enemies may come up to flank you. Get rid of enemies one at a time and close in on the garage but watch for enemies inside. Once there, take cover because one of the shutters on the far side will open and will have enemies beyond. Go through the now opened shutter and take a right. You'll see a bus. Climb inside and make your way to the back. The back of the bus will blow up and you'll be attacked from enemies to the side of the bus. Take them out and then try going out the back of the bus again.
Once you've exited the bus, walk forward and a building will fall apart in front of you and squash a couple of enemies. If you manage to kill these enemies at any point before being crushed (even before the building begins to fall), you'll get an achievement/trophy. Once that's done, head through the door on the left. In the hallway, a couple of enemies will enter from an open door on the right so be ready. Kill them and head into the next room and down the stairs. Exit the room the way you had entered it in the previous mission. Head down the hall and meet up with an old buddy. Follow him. Once he stops on a corner, you'll actually have to move forward or he'll not do anything. Inspect the kitchen area beyond and move near the shutters. He'll open the shutter.
Once into the alley beyond, follow him to the group of soldiers in the streets. Hop up on the gun of one of the Humvees. Start taking out as many enemies as you can until you are forcibly removed from your vehicle. Head back and retreat to your transport.
