Stunt Master Achievement
Finish the training course two days in a row during an Xbox Live Co-op match.
There are two things that you have to consider when approaching this achievement. The first is that you need a partner that can last through the first two levels. You can get this through a private match with a friend who can play a little.
To get through the obstacle course takes patience. The achievement does not call for insane stunts or great bonus times. I recommend that you use the top branching path as you see more of it and to avoid unnecessary jumps.
Practice this in single player until you can consistently get through Monday and Tuesdays obstacle course.