One of the easiest achievements to get. In fact, if you don't have the toybox DLC, the hardest bit of this is getting the agency helicopter!
I think you have to have a certain level of either agility or driving stat before you're allowed to take out a helicopter. They're located on the Agency Tower island just next to the drop point on the island. The downside is they're fairly high up, so you've got a bit of climbing to get to the helipad.
If you've got the toybox DLC, obviously you can just choose a squad helicopter as the vehicle you want dropped beside you!
Anywhos, once you've got yourself into a helicopter, fly over to a Freak infested area. Get yourself a reasonable height off the ground (remember, you want to do a ground strike, so jumping from a metre up is no use), make sure you're above some freaks, and press 'y' to bail out of the helicopter, followed quickly by 'b' to initiate the ground strike.. Assuming you were above a freak when you did this, you're almost certainly gonna see the chievo pop...