A squad of hunters is attacking. Hide behind the middle panel with Alyx while the hunters fire into the wooden boards and blow them away. If you feel brave, then lean out and throw a grenade. Otherwise, stay back for about 4 rounds of fire. Alyx should then switch from blind fire to a normal attack. This is our cue to jump out and fight.
Here's a quick lesson on hunters. They are tough, fast, and fire darts that cause about as much damage as a pulse rifle, but then explode to cause extra damage. They're really nasty. So this is our chance to get good at killing them. When you jump out, draw the SMG and fire 3 SMG grenades into one of the 2 hunters. This should kill it. That makes the fight much more manageable. Hang back and help Alyx kill the second with fire from the magnum and the pulse rifle.
Push forward to trigger the third one. We're going to learn a new trick for this one. You can just pound it with the shotgun, or you could use its own darts against it. Pick up a tire with the gravity gun and use it as a shield. When a few darts hit it, throw it back at the hunter to set off an explosion. It will do a lot of damage. Do it once or twice to kill a hunter. It's also worth an achievement.
Head on outside and break the boxes by the door. Another 2 hunters are attacking. There's one at the front gate and one at the back. We could do another series of cat and mouse fighting, or we could jump in the car and take them out. Yes, the most effective weapon against hunters is actually the car. Just hit them with it. The speed you're going will determine the damage. You can also hit turbo right before you reach them for a guaranteed kill. Just run these two over from the relative safety of your car, and then jump back out.
Go back into the transmitter building and wait for Alyx to make a fairly fruitless call to White Forest. The back gate is now open, so jump back into the car and move forward. Follow the dirt road and go between the boulders. You should be able to just follow it forward until you fall off of a cliff and hit a loading screen.
Drive down the road and pass by the Combine wreckage.Park in the middle of the area and move toward the barn on the left. Go inside to the cage area. Shoot off the lock and open the door. Go ahead and follow horror movie logic. Drop down into the hole by the 2 dead rebels to find the advisor pod. That's the thing causing the pulses. Alyx should expose the core. Use the gravity gun to knock it out and trigger some unforeseen consequences. Just ride it and wait for the big finale.