A way to grind this out by yourself:
- Go into multiplayer and create your own lobby with a password on it - which stops anyone else joining unless they have it. Select which ever track you prefer the most.
- When arrive at the track by yourself, just create contests as usual, just need to make sure they are DUEL. This type allows you to race by yourself with the clipping points and clipping zones - so good for those achievements too.
- You won't get any extra cash or EXP for these races, but you will get some when leaving the lobby based on your drift score as per usual.
- This will still take a long time to do even by yourself. The shortest I did was approx 40secs per race.