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If you havent unlocked it via normal gameplay this is one way of getting Stuntman

You want to select Go Off-Road > Quick Event >
MODE : Simple Race
TRACK : Wild West
AI : 1
Boost Madness - YES
Collisions - NO


I let the Ai get a lead before attempting this. I am aware that collisions are off but i found it easier with them gone.Boost from the starting line,down the right path and take the right jump here In roughly the middle left from your jump you should be able to see a Boost ring encased in a rock circle. Your aim here is to pull back with left stick whilst still full boosting and head to the centre of this ring,you may need to balance a little with the LS, and hopefully sail straight through,then try to keep your jump travelling as far as possible. Im not sure if you need to go through the ring for the achievement im guessing not but this is how i unlocked Stuntman When landing i didnt crash not exactly sure if this matters.If anyone can shed more light via comments ill edit the solution

One thing to note the achievement took about 7 seconds to unlock, not sure if thats due to my USB drive but if after a few seconds it hasnt unlocked restart the race

There are other maps to obtain this achievement, so i advise playing the tournament and if it hasnt popped use the method above. With the other big jump maps they are later in the level, but with the one i advise its pretty the 2nd jump you do therefore if you fail its a quick restart and after 6 secs you are back to the same point