On the level Tsavo Highway, after the Rally Point Alpha checkpoint, there will be a battle involving two waves of brutes and then a wraith. In the buildings in the back of the encounter area, there are 3 trip mines. Your best bet is to grab either a brute shot or fuel rod cannon off the bodies of the brutes and shoot one of the choppers in the next area until it is almost dead. Then, lure it towards you and at the last second, throw down the trip mine. This will kill the chopper and earn you the achievement. Make sure you play on easy, for obvious reasons.
Something like this: