Reportedly, this has been patched. I will leave this up in case another exploit is found and I no longer have the game to confirm the comments.
https://www.trueachievements.com/DarkxMaterials.htm has said that this achievement can be buggy. This is what he has said.
Kraven PL has informed me that you need to be Rank 8 to access the required track. www.ign.com/wikis/gta-5/Races_(GTA_Online) You can get this achievement in GTA Online as well as single player.
I got this achievement while grinding out the Downtown Underground race [online].
I did the same stunt jump apparently fifty times over a few days. This was not in one session nor were different stunt jumps used, so it is cumulative.
I didn't do all the stunt jumps, just the same one fifty times.
Basically, I was in GTA Online grinding out the Downtown Underground race. In the race, you have an opportunity to get a stunt jump because it is directly in the race path. Each time the jump is successfully completed it will award xp and apparently add one to the counter controlling this achievement. When the [counter] reaches fifty, the achievement unlocks. This does not need to be done in one sitting because I got the achievement early on in my session last night.
He is unable to post, so I have posted on his behalf. Here is the original thread where he mentioned this.
You can also find another thread, on
X360A and follow that thread for more comments.
UPDATE: DarkxMaterials has added a photo and a video to support his claims
Update 2: AssassinGnome has commented that you can go up to the ramp and then head straight back and do it again instead of 50 races. Of course, you do not get the Reputation Points for doing 50 short minutes.
Thanks.