Drop in on any mountain and look for any challenges with a red flag next to the challenge icon. Once you have found the challenge you want to complete and claim the achievement on, start the challenge and check the leaderboards for the high score. Also, check the sponsor icon next to the name, make sure your sponsor is a different sponsor. Beat the score on the leaderboard and the achievement is yours, if it doesn't unlock, exit out to the main menu and refresh the sponsor. If it doesn't unlock, try again. It is possible that someone else may beat your score while you are setting it, so check the leaderboards more than once.
The leaderboards are reset weekly on Sundays at 4 AM GMT.