You must win a total of 201 races, simple. So here is a simple way, the way that I did it. If you just want to get this achievement go to Bristol (6laps) lowest possible, (2opponents), plus everything else on the lowest possible like opponent difficulty etc... Race over and over, it will take 2 to 3 minutes per win. But this achievement is an accumulation of all wins
If you are going to use this method only, it will take at most 10 hours
to complete. Once you get good at it, it will take at minimum a little under 7 hours
to complete. It might be a better idea to play the career and get other achievements at the same time.
*** For me at the time I was racing to get the highest achievement score possible and I just wanted to snag this one up quick and come back to this game in the future for the other achievements. This was the method I used.
If this method has helped you great, if you decided to go through the career instead that is good also!