I would suggest the 2nd race, set to easy. To complete this challenge I used the Koenigsegg CCX-R. At the start of the race get a shield and nitro before the tunnel. The hardest turn is the S-curve. If you stay to the outside initially you can go almost straight through the whole turn. If you hit the wall then just use your Nitro before you loose to much speed. The game will let you know when you finish a whole lap going 120.. You can start your '120 lap' at any point in the race, you just have to get back to that spot without going under 120...and if other racers are causing you a lot of problems either try to burn them at the beginning and get nothing but nitro's until you have a good lead, or let everyone get out in front of you.