Once you get to level 50 they throw RNG into the game. From now on after hitting Max level there is a 4th medal to all the skills, Platinum. It looks to be that Plat is only slightly farther than Gold and attainable with the better cars. Your best bet is picking the best car in your chosen class and ONLY do the Jumps. They can be done over and over so you get more chances per hour to unlock that 5% level 50 part.
Once you unlock all plats for one car the achievement will unlock.
The only one I had a problem with was the Exhaust as there are only 3 jumps for it and they are all on dirt roads.
If you're going for the plats with Raid cars then use hill climbs instead and follow the world record holders. They go off the off beaten path to get to the end quicker than taking a normal route.
UPDATE: so with the patches that don't add any achievements to the game but still charge 20 bucks a pop they've raised the levels of the cars. as confirmed by TakiTato and found out by UnfurledEmu75 you HAVE to have the parts be under level 50