CRUSH letters appear throughout all the Hill Climb events: Time Attack, Stunt, and Survival. To unlock levels for Stunt mode, you'll need to play through them in Time Attack first.
Every Hill Climb level has five letters (C, R, U, S, and H) spread throughout the level. You must try to collect the letters and then complete the level. Medal rating doesn't matter, but you'll only keep the letters if you cross the finish gate at the end of the level. Die or restart and you'll have to pick them up again.
You don't need to collect all five letters in one attempt for this Achievement. Just play a Stunt level until you get them all. This might require managing your speed a little, slowing down or reversing to get hard-to-reach letters. Give a level a few tries and you'll learn where everything is.
Pause and choose to restart if you miss a letter while going for this Achievement. Or don't worry about it, because you're bound to get all five at some point as you progress through Stunt mode.
Progress towards the letters Achievements can't be tracked through the Achievements app, unfortunately. But it shouldn't take terribly long to get them all.