We descend into the canyon. Below are several antlion repellent devices. We move from one to the next, shooting the insects. When we reach the last device, protected by a fence, two guards will attack us from the mines.
The guards can throw cars, carts, and other large objects, so it's best to stay inside the fence. We use the Gravity Gun to pull explosive barrels and cylinders toward us and throw them at the bosses. Only then should we start wasting ammo. This is an unfavorable position for the enemies, so sooner or later we'll be able to destroy them.
After avoiding all the traps, we jump onto the elevator roof, pull the bolt, and lift the hatch cover. The mechanism in the elevator cabin is blocked by a pickaxe; we knock it out with a gravity gun pulse. The elevator will fall, we board the passengers, and we head up together.
Up in the checkpoint room, no one is alive; all the rebels have been killed. We pick up a pulse rifle and some first aid kits along the way. We reach the observation deck; one panel is activated here, and there's a protective field nearby. (From the deck, we can see G-Man standing in the distance, on the near side of the destroyed bridge.) There's a car on the other side of the bridge; we need to reach it to outrun the Combine. We head there alone, with Alyx providing cover with a sniper rifle.
We jump down, break the boards beneath us, and fall further. We pass through the wooden wall. In the room, we are attacked by a zombie carrying headcrabs; we throw grenades at it.
We fall further down, finding ourselves in the cabin of a non-functioning elevator. We move along the open floor, fending off poisonous headcrabs. Here, Alyx will begin to assist us, shooting the monsters.
In the narrow corridor, there is a fire trap that the zombies are trying to pass through. We turn the valve to shut off the gas and pass through the extinguished fire. We can go back, turn the gas back on, and fry the zombies pursuing us.
We go outside. A fast zombie will jump out of the dumpster. Ahead is a locked gate; to open it, we knock down the padlock. Around the corner are numerous zombies, but this is a fireable area; Alyx will destroy them all.
We enter the warehouse with shelves. We crawl under the first shelf, crawling between two rows. On the other side, we climb onto the upper shelves. We need to reach the upper balcony; there's a car lift in front of it. Lower the lift platform, place boxes or a refrigerator from the adjacent shelves on it. Raise the platform, and use the placed items to jump to the balcony.
Exit through the ventilation. This is a pool with toxic waste; you can't go into the water; you need to jump across the islands. Numerous zombies will emerge from the water one by one. There are numerous regular barrels nearby that you can throw. If you haven't yet earned the "Bonebreaker" achievement, now's the time.
We drive the car to the left wall, climb this wall, walk along it, and jump to the central span. The bridge is swaying like a seesaw, and we need to tilt it to the other side. To do this, use the gravity gun to push all the cars to the side that is currently higher. Some cars can simply be thrown down. The bridge should tilt in our direction.
We jump down the hill and return to the car. We get in and drive, jumping over the two resulting cliffs, and find ourselves on the road near the starting checkpoint.
We enter the side room, press the lever, this will open the doors for Alyx and the Vortigon. The alien lifts the gate on the road for us, and Alyx and I can continue on.