I thought I'd make a contribution for this mission, because it took me a while to figure how to do it.
Ok, so when you start, drive around the stage and collect as many coins as you can, you need 50 to complete the mission. After a few seconds, king boo will come at you. There is no way to dodge him, so don't waste time trying. When he hits you, you will lose most, or all of your coins. And the screen will go dark. Then, you have to chase king boo and drive into him, then you will get the coins back that he took off you. Once you have them back, drive around the stage and get more coins. Keep doing this until you get 50 coins.