Pretty easy, i found a garage next a repair shop, i attracted a cop there, and I have drivei n circle and use the shock wave on cop, you should have more cop quickly if you stay in the zone.When my car was too damaged, i passed in the repair shop, to repair my car and reload my shock wave.It took me 20,25 minutes.You can also unlock the trophy "Multiplier".
Or you can do as in this video.(credit to Throneful), but you have to wait that your pilot level increases, I am level 12, and this race is blocked for me.
You can also unlock the two trophies "Multiplier" and "Bigger points" with the video solution.