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To unlock Dry Bones, you'll need to beat Leaf Cup in 100cc mode.. If you're fairly experienced at Mario Kart games, try the popular Mach Bike. This is a great vehicle, but its low acceleration can punish you if you get hit or run into a wall. If you're having trouble with the Mach Bike, pick a bike with better acceleration, or just choose the bike you have the most experience with. Characters have a smaller effect than vehicles, so pick the one you play most in your chosen size category. Your goal is to get 1st place overall for the four tracks of Leaf Cup. You can still win if you get 2nd or 3rd on a race, but if you drop too far in the standings, quit to the main menu and try again. If you haven't unlocked Leaf Cup yet, play Banana and Shell Cups. Once you have at least 3rd place in both of those cups, Leaf Cup will unlock.
Start a 100cc Grand Prix game. Choose your character and vehicle. Choose Leaf Cup.