Blur

Shannon is the "boss" of the first set of races. You are given a list of tasks to accomplish in order to unlock the race. The game keeps track how close you are to unlocking her race on the race select screen.
The tasks are:
-Finish 4 races with 4 different vehicles. (You don't have to come in first place, just finish 4 races with 4 different vehicles.)
-Earn 7000 fans. (Fans are earned through the races. The better position you finish, the more drifts you pull off, the more opponents you hit with power-ups, the more fans you will get.)
-Win 5 races. (self explanatory)
-Land 80 offensive power-ups. (Use the power-ups found around the tracks to attack your opponents and hit your opponents 80 times. It may sound like a lot, but it comes very quickly and naturally through gameplay.)

Please Note: There may be a Glitch with this achievement, hence the fact, I had completed all of Shannon's Set Tasks, yet the achievement did not unlock.

To get the achievement, you may have to delete your Save File off of the Xbox Hard Disk Drive, load the game, and start a new Save Game.

UPDATE: I have just tested that Achievement, and, unfortunately, to unlock the Achievement, I had to delete the Game Save File off of the Xbox 360 Hard Disk Drive, so, please note, that, if this Achievement glitches for you, you will have to delete the Game Save and start all over again.

To delete the Game Save File:
From the Xbox 360 Dashboard, go to My Xbox -> System Settings -> Memory -> (Save Location) -> Blur, and delete the Game Save File that is within that folder - deletion of Extras like Content Packages is not necessary.

Note: You have to delete the Game Save from the Xbox Dashboard, because it cannot be deleted from within the Game.