*Edit: Props to Vx L ROCK
for confirming that it has to be 'Awesome Air' skills. Having just popped this I would recommend using the hills around Redstone Gold Mine. What I was doing was either going slow and aiming for smaller hills or if I hit a bigger hill brake around halfway up so that the overall jump is smaller but is long enough that it doesn't qualify as a 'Great Air' skill. Video below of the exact area I used.
No specific 'Awesome Air' skills challenge has appeared in the past, but various other variations of either 'Ultimate Air' or just regular 'Air' skills have.
For reference an 'Awesome Air' skill is getting all four tires off the ground in any, shape, or form, but for a longer time than either just a regular 'Air' skill or 'Great Air' skill, but less time than an 'Ultimate Air' skill. 'Awesome Air' skills are required and 'Ultimate Air' skills will NOT count for this challenge, as mentioned in the edit.