You have to complete all races and events with gold medal in Flatout mode.
To win easier in races:
Upgrade your car as soon as possible.
If you low on money here is a trick:
- start a championship with 2-3 races
- try to wreck as many of your opponents as you can because you get much more money for wrecking than winning, position does not count, just finish the championship to get credit for that money
With this trick you can easily get a lot of money and you can buy and upgrade the car that you want.
To win derby:
- pick a car with heavy weight
- pick up every extras on the playground (heavy weight, bomb, etc.)
This achievement will take a little long, but it is not that hard.
Good luck for getting this achievement.
If you can add some extra tips, tell me.
Edit1: shotgunbilly555 mentioned "During time trials, the time doesn't start until you cross the finish line. You can back up and get a rolling start to help shave some seconds off your time"