The easiest place to get this is in the chapter 'Lowlife' in Half Life 2 Episode One, so start a new game there if you aren't there already.
Play normally until you reach the first antlion burrow. Now, instead of hitting the car over the burrow to stop the supply of antlions, keep using the gravity gun to whack the car into the antlions one by one, which will crush them. Repeat 30 times (it sounds a lot, but is actually over with surprisingly quickly) and the achievement should pop up immediately.
If Alyx is getting too many of them you can try in the next area (cover up two holes but leave the last to get the achievement).
What's great is you will get the 'Car Crusher' achievement at the same time!
So that's two achievements in one.