Get 100% Story Mode completion with any character. Edwards car is extremely easy to use. You can go back to previous races and improve if you didn't manage to get Gold etc.
After doing the above you will be able to select "Xtra Crispy" the next time you start a new Story Mode game. This new mode is basically "Hard" difficulty with slightly tighter time constraints, higher scores to beat in Style Events and tougher opponents in Duels and Races. Again, I advise to use Edward's car.