There's a chopper chase challenge in "Dry Rigs" (one of the early areas you can unlock) that has a steep cliff/drop-off on the right side of the course as you get near the finish line: Simply hit top speed in the quad you're driving and go over the edge; you'll get the achievement before you hit the ground.
Keep in mind that if you drive off the cliff you have no chance of winning the chopper chase, so you'll have to do it again. Luckily, it only takes about one-and-a-half minutes to do this challenge.