i got this one simply by jumping Piss Handle Gully (or whatever it is called). I landed half on the ramp and tumbled a few times. It must have counted the tumble as air time, as the achievement popped up on my second roll. My co-op player got it thanks to my bad driving. I drove up on a rock and got stuck. I got out to punch the vehicle off the rock and he stayed in the turret. When I punched the car off and it rolled, he got the achievement