Okay. It's time for a solo mission while Alyx fixes up the car. Look outside through the door to see the autogun. This thing can set a zombie on fire with a single shot, so we're going to have a little fun outmaneuvering it. Follow our rebel friend up the steps and then drop down the trap door to the office below. The zombies shouldn't be able to break down the door. Carefully break away the boards from the window and then use the saw and the 2 oxygen tanks to clear out the zombies near you. Go ahead and use up some pistol ammo to pick them off. There's no rush, so don't waste the good stuff.

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Once it's pretty clear, jump out and get ready for a bit more fighting. Use the gravity gun with some barrels and tires to knock out the zombies and zombine that emerge in front of you. Your goal is to get up on the truck, so you don't have to kill all of them. Get in it and use it as a step to get up to the roof. Break the window and jump inside the office. Open the door to get out on some exterior stairs. Jump off them and get onto the clean land.
The zombie over to the side should be more interested in the autogun than you, so just ignore him. Go up and over to the left side and go through the little trench. Crawl through the pipe and blast the flaming headcrab that comes through. Keep pushing forward, but stay by the barrels and stay crouched. If you stand up, the autogun will nail you. There's a normal turret covering the end of the lane though.
You have 2 good choices. You can toss a grenade from the relative safety of the barrels, or pick up one of the barrels and crawl forward while you use it as a shield. Then just get up close and launch the barrel. Stay low and approach the office at the end. Once you're there, stand up and fire into the little building. The truck by the window will keep you safe from the autogun, but there's a combine soldier and shotgunner inside. Kill them and jump into the building. There's another soldier outside, so gun him down and start moving along the second row.