Simply beat all eight hard tracks which is hard but possible. You need to beat most of the medium levels to unlock them. You will have to complete the stages within 500 faults and 30 minutes.
Practice the move that you learn in the hard tutorial as you will have to use the hop move expertly to get through some of the harder parts in these stages. When going up hills, take it slow and ease on the throttle, that seemed to make a lot of those impossible hill climbs doable.
Look at the leaderboard videos for help on the sections that you are struggling with.