Everyone knows the best weapon in a first-person shooter is Half-Life 2's gravity gun, simply because it has never been so fun to tip cars and filing cabinets alike on your hapless opponents.
Fun G-gun related hint: use it to carry Ball (from when you first get the G-gun and meet DOG for the first time) into Ravenholme.
Why? Ball is Combine-allied, whereas those zombies and headcrabs are all headcrab-aligned. Both are stupid, and hostile to each other. Throw Ball at some advancing zombies, and not only will he kill one of them, but then the rest of the zombies and any headcrabs in the area will attack it, and Ball will fight back. It is also invincible. The zombies and crabs will just stand still and smack at it and achieve nothing. Ball won't damage the zombies, but he can kill headcrabs, and keep the heat off you, leaving you free to line up some juicy headshots. Drop him down the mine shaft to tie up and kill a huge number of headcrabs as well - he'll take down nearly all of them in the time it takes you to get down there.
Be warned - like all rollermines, he explodes when he gets wet, so you won't be able to take him past the flooded section in the mine, and even if you DO manage that, you can't carry him in the jeep.