I never got the achievement right at 20 wins. in fact I was at 41 wins and achievement still didn't pop for me. what I did was I went to the storage to delete every game save I had for midnight club.(don't delete until you know you have all single player done or if you wish to start your progress all over). then I counted every ranked win I won after deleting game saves and pop got it right at 20 wins.
Once you delete your game saves leaderboards will remain same. don't worry about that, just count off your 20 wins