Half-Life 2 Walkthrough -> Through the Channels
Kill the next soldier with the same pistol, aiming for the head. Missing shots or shooting at the body will quickly run out of ammo, but this way you'll have plenty. After that, head outside. Go down the stairs behind the fence (don't stop completely on the first landing).
A train will be arriving soon. As soon as it's within jumping distance, jump onto the roof and cross to the other side. Turn around and shoot the barrel, blowing up two soldiers. Quickly grab your crowbar, climb the stairs, and knock out the boards in the doorway.
Run down the corridor with the crowbar at the ready and take out another soldier on the stairs. Still holding the crowbar, cross the narrow bridge, smash the crates blocking your way, and carefully shoot the two soldiers entrenched below on the rails with your pistol.
After that, it's worth healing up and grabbing the "batteries" in the alcove on the right, then, running to the canal, jump into the water. That's it, this fight is over. It wasn't all that difficult.
The train car's occupant will show you the way, and the Vortigaunt (a large, humanoid monster with a single red eye) will recharge your suit. In the first part, upon leaving the lab, they were enemies, and very powerful ones at that, but in the second part, they're best friends.
After a short but intense fight with two pieces of wood, you'll encounter a remora for the first time. While it's eating a crow, approach its tongue. When it drags you toward it, draw your crowbar, point it at it, and keep hitting it (the exact moment is easy to predict, but it's best not to risk it). Don't kill it with a pistol under any circumstances—it'll use up a lot of ammo, and with a crowbar, it only takes one hit. Hear gunfire? Ready your pistol and follow the sound.
Keep your pistol and flashlight ready—you'll have to deal with a soldier in the pipe or the hiding place. Collect supplies from the hiding place, reload, and quickly get out.
Destroy the barrel, blowing up the (almost) defenseless riflemen, and then take control of the machine gun. Ammo is endless. It'll be fun: the key is to time your energy spit attacks on the soldiers and scanners. Be sure to detonate the barrel near the wall on the right—it's on a platform blocking the passage along the wall.
Lots of Barnacles
After a short fight with two soldiers (you can use the bridge supports for cover), enter the narrow gap in the pipe to the right. Be careful—the ground is slippery and there are a lot of Barnacles, but they're easy to kill. Treat the next large group of Barnacles with a flammable barrel. While they're passing it around, light the barrel and enjoy the fireworks.
Make sure your health is at maximum. Don't jump into the water too quickly—let the burning barrel pass first. Several more will follow—hide behind the plank and don't forget to catch your breath. Carefully eliminate the Combine at the bridge.