The kid gloves are off now. Gordon is really going to have to fight it out with the Combine. Your goal is to take on the many Combine positions and help the rebels advance and take the Citadel. For now, you just have to try and find Barney and his main attack group. This level has you taking on a whole bunch of enemies and a few physics puzzles along the way. You'll also get to team up with Alyx for a defense mission right before the end. You better be ready for a long fight, because it only gets harder from here on out.
"Protect" Dog - Yeah...just watch Dog clear you a path.
Link Up With the Rebellion - Gain your first squadmates and do the early training to advance. Once you order them around, you'll come under a basic attack.
Fight Through the Hopper Field - Some headcrabs and a whole lot of hoppers in the field. Have fun learning the easy way to deal with these guys. They're scattered through the rest of the game.
Reach the Barricade - You have to breach the barricade and do a bit of a flanking attack.
Disable the Barricade - An easy attack once you get behind them.
Clear out the Combine Base - This heavily fortified office is right behind the barricade. Be ready for a lot more close quarters fighting.
Clear the Car Park - FIght through the endless waves of man hacks and get to the back of the Combine position.
Defend the Car Park - A bit of a standard defense mission. Hold your ground and try to keep your squad alive while the rebels clear the debris.
Open the Locked Door - This requires one long side path through zombie infested toxic water. Have fun!
Escape the Rebel Base - Your sanctuary is already under attack. Fight through the series of buildings to escape the Combine attack and reunite with Alyx.
Help Alyx Destroy the Generator - You'll have to keep Alyx safe while she hacks the generator's shield. Just keep your head down and stay in a safe spot while you hold out against the Combine attacks.
Navigate the Sewers - The sewers are filled with zombies and Combine. Let the two kill each other off to make the actual fighitng limited.
Clear the Factory - Your last goal is to get through the factory. The elites might prove to be a challenge, but you're almost home free by that point.