Not too difficult really. Just have to finish up the Knife Flights and Under the Bridge challenges.
Sadly for the separate Stunt Jumps achievement which requires you to successfully jump all 50, I went through a guide and completed them all... or so I thought. According to the statistics in-game, I've found 50 of them, but only completed 47, after being sure I didn't move on to the next one until I got the yellow "Stunt Jump completed, ___ amount remaining." Wish Rockstar would add a stunt jumps section to their website to show which one's you've missed.
