Almost all guides online suggest that you use the NRG to get this achievement;
I disagree. The reason they suggest this is because you don't have to level a skill in order to complete it, you can just drive to the spot and do the challenge. However the NRG challenge is significantly harder than the BMX challenge. There is nothing wrong with attempting the NRG challenge but if you try it a few times and are struggling I suggest doing the BMX instead. Here's why:
1. You need a cycling skill of 50% to even start the BMX and realistically a 75% skill to successfully complete it which is why other guides advise you to steer clear. However it doesn't take long to raise your cycling skill. Simply grabbing a bicycle and riding it around the outer edges of the map 1 time will give you enough skill to complete the challenge. This takes approximately 15-20 min.
2. BMX challenge is super easy once you have the required skill to bunny hop
high enough to get the coronas.
3. While leveling your cycling skill your stamina and max health will also increase.
4. All of the skills in #3 will make winning the
Beat the Cock trophy/triathlon significantly easier. (Although getting $10k in burglaries for unlimited sprint is still recommended for triathlon.)