Only "Air" skills count here, nothing with "Great" or "Awesome" or "Ultimate" modifiers. One easy way to get it quickly is to use a fast car with low center of gravity... Lamborghini Centenarino comes to mind because of default 4WD, but if you don't have it, any will do.. or ultimately just about any car will do, slower ones may just take a bit longer.
Anyway, just start driving back and forth, offroad across the northern outback at max speed. You should get Air skills pretty regularly and knock this out quickly because of the small bumps and imperfections in the brush.