I recently earned the Half-life 2 achievement “zombie chopper”. The description is “Play through Ravenholm using only the Gravity Gun”. So I decided to share my expertise! The following is a list of tips that should help you get it (I did… first try :P)

Know the maps
Make sure you understand where you need to go in every map and how to get there. The faster you make it through each map, the fewer zombies can attack you.
Quicksave often
Save every time you clear an area of zombies. Remember, just the act of shooting, no matter where, voids the achievement. Constant quicksaving also allows you to go back to where you were if you made a mistake. Health is fairly plentiful, but it helps to be attacked less.
Grab the first sawblade…
And carry it everywhere. “Trusty rusty” is very helpful as it doesn’t explode and takes up a tiny portion of your view. You might lose it behind zombies but there is always another nearby in those situations. It’ll stop you sprinting but that doesn’t really matter. Walking and bunny hopping will keep those zombies away… they’re so slow. And sprinting doesn’t help against fast zombies, thus it’s not necessary.
…or grab the chopper blade…
This is something I was experimenting with recently, and I’m not really sure how good it is. First of all, it can be found soon after the first chopper (you’ll need rusty to get this far). After you see that in a small alleyway, you come into a building with another chopper where you can hear Grigori. Leaning against a wall in that building is an unattached blade. It’s similar to the sawblade in that you can grab it with the G-gun and throw it at zombies, however the advantage of it is that it’s bigger so chops more zombies at once, and is easier to find and grab after throwing. Although, there are disadvantages too:
It is wider than doorways and will always be perpendicular to you, so to get it through doorways you have to stand at an angle and slide it through, then walk through. The trickiest place to get it through is through the trapdoor shortly before the church. You have to get at an angle and then keep it against walls to make sure it’s upright. Or, you could just ditch it and use the sawblades in the graveyard.
It can bounce back and kill you in one hit, or even if you’re just struggling with it on a narrow passageway it can unexpectedly hit and kill you. When carrying the big blade, quicksave even more often.
…or grab a radiator!
You’ll still need a rusty to get to a radiator, but they kill zombies in one hit, are very big, and are much easier to locate after hitting something.
I guess really you can just grab anything, Ravenholm was called “Phystown” at one development stage for a reason 😛
Play in easy
Easy mode makes it *equips sunglasses* easier.
After getting the shotgun, dive
Although, really, you could jump into the water tank before that. Not like the shotgun’s much use 😉 Press the elevator button ASAP and hit the zombies as much as you can with rusty. Run out of the lift once there’s room and straight past the poison zombie – he’s not worth your time.
Waiting for the trolley
When Grigori’s preparing the trolley, put both medkits on opposite sides of the trapdoor. When you’ve lost 25 health, go for one. The ideal strategy for avoiding the zombies is hit one or as many as you can at first with trusty rusty, then running around in circles avoiding the zombie’s hits. They don’t hurt much so you can survive, and try to jump into the crate before it’s at the ledge (halfway is easy) to avoid them more.
Have patience
In parts where you have a height advantage and there are lots of headcrabs/zombies below, pick them off slowly, shining your flashlight down alleyways to attract zombies. Always quicksave before jumping down in case you get hurt or neglected zombies.
Go for lambdas
There are a few lambdas that aren’t “guarded” and contain batteries which are quite useful.
Wait until you’ve exited the mine
The achievement won’t unlock until you’ve exited the mine, and can take a while to unlock. Be patient for a few seconds and you’ll be ecstatic to start swingin’ your crowbar.
In the mine, stay above the headcrabs
There are too many to fight, so just make sure you’re always above them. There are few occasions where you meet them, and walking quickly and unpredictably will keep them away. It is actually possible to avoid them completely though: after landing on the catwalk, you might see a platform resting on a single support. Sprint to it (you don’t need your blade anymore) then sprint off immediately before it collapses.
Once you’re on the cave supports, stay on them for as long as possible.

If you’re careful enough, the achievement is easy. Be sure to use trusty rusty, move quickly and stay out of harm’s way.

Hopefully this helped!