Win a race against a player on any leaderboard to unlock this achievement.

For the 5, 25 and 100 versions, win 5, 25 and 100 races.

Racing against leaderboard times is probably the best way to get faster at Mad Skills BMX 2. To earn this achievement, open any leaderboard. 

Next, race against anybody on the leaderboard you've chosen by tapping the green play button. You'll get faster by attacking players just ahead of you and working your way up the leaderboards.