Enjoy the little scene in the office. Once it’s all over, just follow Alyx into the elevator. Ride it down and run toward Breen. He’s runs off without the gravity gun, so pick it back up. Wait for Alyx to figure out the Combine systems while the elevator returns. It looks like Breen’s making his grand escape, so we have to stop him.
Get onto the elevator when the doors open. Ride it down and into the teleporter room. Wait for Breen to finish his speech. The piston next to you should start moving near the end. Go across the narrow ledge while it’s up. 3 Combine run out of the forcefield. Just nail them and advance to the next set of forcefields up ahead. You need to run through all 3 of these forcefields by pulling out the 3 energy cores. Be very careful, since bumping into the core means a quick death.
Enemies will come out ahead of you and behind you. Use the cores to take a few of them out. You can ignore them for the most part though, thanks to all the shielding from the glass. Once you get to the end, just step onto the piston and ride it up. Blast the 3 Combine waiting up here and move forward.
Breen should start his ascension around this point. This is a bit of an actual time limit. That said, it is so lenient that it basically shouldn’t matter. Go past the large piston and go to the end of the glass ledge. Step onto the platform as it descends to you. Ride it around and step off onto the higher platform it takes you to.
There’s a charger up here, so use it to heal up. Kill the new wave of 3 elites, then advance. Step onto the piston in the middle and ride it up. Kill another wave of 3 Combine and then wait for the large piston to fall. Step onto it and ride it to the top. The portal opens as you do this.
The Final Battle
2 Combine gunships teleport in through the red portal. Use the gravity gun to pull out energy cores. Go ahead and take out both gunships. Just aim at the big propeller. 2 hits anywhere should count for a kill. With the gunships gone, just focus on the portal. Pull out a core and fire it into the green portal.
The shielding comes up after the first shot. Just keep pulling out energy cores and firing them into the core until the whole thing falls apart. When Breen’s elevator sphere falls, you’ve won. Just sit back and enjoy the ending. Congratulations, you’ve completed Half-Life 2. Now get started on Episode 1.