It's not easy to beat the time... but pratice is the key AND your chosen driver. I read the xbox360achievements guide for this achievement, because i didn't saw a solution here. The guide tells your to use Divizioso, but i had a hard time to handle his bike... MY suggestion is to use Randy de Puniet, because his bike was way easier to handle for ME and if you compare both stats, you see Randy have more accerleration, speed and handling. If you have trouble with Randy, feel free to use someone else... my time: 01:19:12 after many attempts! (default bike settings)
maybe you wanna download a ghost from the leaderboard before you start or you use your own ghost and learn from your own mistakes.
however: good luck!