This one is not need
to be done in championship races. Fastest method is: get in 1989 Porsche 944 Turbo and do some head-to-head races against drivatars, trying to not crash. It only takes few minutes, simple as that
If you want do it in championship it's Sportscar championship (thanks to N0BLE 360)
I unlocked it with stock car, HAMbidextrous1 said there is no upgrades allowed, but MattQuin89 and some other players unlocked it with upgraded cars. Choice is yours.