The Orange Box

At a certain point during episode 1, antlions will infinitely respawn from holes in the ground, you are supposed to cover up these holes with the busted cars, simply cover all but one, then pick up the car with your Gravity Gun and as they come out from the hole run the car into them for a kill, do this 15 times and its yours.

There are two good spots for this, near the start of the chapter Urban Flight, or towards the end of Lowlife.
Both areas will have 3 ant lion holes, so you need to block 2 with cars, then get a car near the third hole.
I would stand behind the car, let Alyx kill the first one that came up, then listen for the second one and hit the car with the gravity gun, this would squish the antlion and cover up the hole again, then when your ready you can hit the car again, run around the back of it and let Alyx take car of the first ant lion to come out then hit the next one.
I found this way easier because you are only dealing with one at a time and you'll hit it first go.

I did this on the Lowlife level. I found it easier to complete this achievement by not covering any of the holes. Since the 3 holes will spawn more than any one hole could, you can just go around shooting the cars and you'll have more luck hitting one (or more!) antlions in a single shot. It's a bit more dangerous, but possible, and much faster than trying to line up an antlion behind a car when it's chasing you. (If you have the invincibility code on, it's even easier.)