This really separates the men from the boys. Got what it takes? Use the best cars only (Ferrari F50 GT and McLaren F1 LM). It can be extremely hard in places so you need to keep your cool. Play nasty by swerving in front of your rivals or making them spin on the grass. With enough practise you will get this. When you do this achievement you will definitely have enough kudos to be rank 1 so there will be no need to visit single player career mode again.
Note: If you do platinum on the first run through, you will be awarded all other Champion achievements as they are stackable.