Head towards the shooting gun. There is the switch closing the silo. To get there, you will have to walk around the silo.
When you get there, close the silo.
The entrance on the other side will open and Alyx with rebels will show up. Go there.
You will get back to the lab to watch the transmission that you brought. It will concern the mystic Borealis ship.
After watching it, Alyx's father will try start a conversation concerning you and Alyx. However, it will be interrupted as doctor Magnusson will rush in. You will have to lead a combat against the Combine. Follow him.
Magnusson will show you his very new invention called: Magnusson's Device. Listen to his instructions and practice with the device.
When you are ready, walk to Magnusson (he is right behind you). He will open the door for you. Get inside, take any goods and listen to the soldier. He will explain to you everything. Then, get in the car and drive out.
You will have to defend the base against the striders. To do so you will have to use the gravity gun to shot at striders with Magnusson's Device. When the device sticks to strider's body, you shoot at it and it goes off. First you will have to go to a sawmill. Drive straight, then left and to the end of the railway. You will find the sawmill there.
When you're there, park the car in such way that you can drive off quickly. Then listen to the commander.
The attack will begin during the meeting. You will see a strider coming. It will be guarded by hunters. Try to destroy strider first, then the hunters. When it's done, get in the car and go back to where you came from.
The fight with striders is the last combat of the game. It's pretty hard and everything happens so fast, that you won't have too much time to think. You have to get back near the base. In the vicinity there are two places where you can get Magnusson's device. The first one is marked as A. The other one which is outside is (D). The base is towards (E). Striders and hunters will be coming from 2 directions (B) (mainly) and C. No matter the direction, every strider will be passing D and will be heading towards E. The base will be destroyed when a strider gets to the gate. The tactics to win this battle are as follows. Every strider is guarded by 2 or 3 hunters. They have to be destroyed first. They are faster than strider so they usually are ahead of him. Retreating near the base makes sense, only because there are rebels that can help you. You're not alone then. Some of them get outside the base. They're not much of a help when it comes to destroying the hunters, but they draw their attention. Begin the battle from parking your car between point D and the base. It's important that it's close to the place where you can get Magnusson's device. Don't leave it in the middle of the road as striders will be passing through.
When you get a message that a strider is coming, get inside a hut. Most striders will come from the B direction. It will be easier to destroy the hunters from inside the building. To do so use the rocket launcher or the crossbow. If hunter is coming from C direction, it's better to stand next to D. In the case they come from 2 directions simultaneously, get inside the hut and get ready for a direct clash. Then you should use the rocket launcher or the shotgun. Then move to another strider. When the hunters are destroyed get to the D point.
Standing there, wait for the strider to come closer. Using Magnusson's device on striders is not that easy. The bombs must stick to its armour, and to be specific to the orange part of it. If you throw the device at other part it will bounce. It's hard to hit the target when it's too far, so wait until it gets closer. When the bomb sticks, change your weapon immediately (by pressing Q) and shoot. It may even be the pistol.
Use this tactic until you get rid of all the striders. After repulsing the first wave, there will be another. And then another one - final. Remember to get supplies from the huts in B and D. After many (or maybe not so many) attempts, the base will finally be safe.
When you get the info that all the striders were defeated, get back to the base.
You will meet Alyx. Follow her. An epilogue will begin.