I unlocked this while going for
Basically an easy way is to choose Showdown from the main menu and pick Ultimate Alienation, set the time limit to whatever you prefer, go for 5 minutes if you want to be extra sure this unlocks but chances are you will succeed within the minute. Pick any arena you like and just stand on a power-up, hold the left analog stick upwards and keep pressing "A". You will hit all karts eventually.