Okay I can personally confirm that the best level to do this is the Third Level (Bone Cage). Now you need someone that is a "Jump Rider" and a board with high jump and preferably speed. I choose Pestor and used Pestor's DLC Board (80MSP), and got this on my first try.
A few tips I can give you that no one else could give me is to continually jump all the way to the bottom. This way you can get small yards that really add up (20-30 yards). Now for the main trick, make certain that you jump right before your board leaves the end of the ramp. This really helps out and is the main reason you picked a "Jump Rider".
Another thing that can help out is there are a lot of places where you can turn around and lean into the mountain. This will get you back up it part-way so that you can get a few extra jumps in. Beware though there is a time limit so if you spend too much time trying to grind it the time will expire.