Level 40 is reached at 370.000 points. You should get this achievement by simply playing the carreer.
Generally, if you want to boost your points here's what i did:
- set difficulty to easy
- go to quick race
- choose race, 20 laps, 15 opponents
- track: miyatomi, as it is the shortest
now start the race and you should easily overtake a lot of opponents!
so at least, you should get around 10.000 points in one race which takes about 5 minutes! good luck! :-)