If you, like me have completed 30 races, but were more interested in winning than stunting, a great way to boost is on lighthouse loop. There are two jumps a lap with big air, which, done right can get you 3 stunts a jump, making the total 18 jumps for a 2-3 minute run.
Unfortunately the games tracking isn't the best, but an almost full proof way to get 3 stunts per jump, is to first lean forward until the car is about half way through the flip, then lean to the side with your arms wide and turning, this should also net the barrel roll and spin stunts.
The achievement will unlock mid race when you do your 500th stunt.