I think it is pretty clear from the challenge description that you have to "Perform 20 Awesome Air Skills" and that Great Air and Ultimate Air do not count. You could just drive around over some bumpy ground for a few hours, but here is how I got 20 in a just a few minutes.
Drive around until you get an Awesome Air. Ideally you want to being going in a straight line, not very fast, and under maximum acceleration. As soon as "Awesome Air" appears on the screen press
twice to rewind twice. Hold down the accelerator, drive in a straight line and then as soon as "Awesome Air" appears on the screen again, rewind twice. There is no need to wait for the Awesome Air skill to be banked.
You don't want to be rewinding then having to turn and break and accelerate to try and get another Awesome Air, just keep going until you get a consistent Awesome Air you can repeat every time.