Alright, there's a lot of good advice given here, however they don't say what race to do.
I was able to knock out this and the "Underdog" achievement in one go. What you'll want to do is buy the VW Golf GTi Mk2 and race in the "Hot Hatch Runoff" playlist in the "Proving Grounds" tab. You'll want to select the first race there. You have to be a lvl.3 to get into that playlist, but it doesn't take long to get there. also you'll want to set the difficulty to the lowest. By the end of the first lap I was in 1st place.
Good Luck, I hope this helps.