I have gotten this in "Freestyle" relatively quickly by redoing a challenge called "The Space Jump" (or something similar).
This is also a challenge from the Ubisoft-Store and you can be rewarded with pants and a helmet surpassing 25,000 points in this particular challenge.
I achieved this best by taking a snowboard and making barrel rolls when jumping over the ramp. You'll likely drop down a thousand feet and the landing is a piece of luck, but if you can make it, it will net you between 16,000 to 25,000 points on each jump. (to ensure a more stable drop, you should grab the board by holding both
at the same time)
After a few attempts, I was promoted the legendary rank in that category, also getting four to five level-ups in quick succession.