After you make it to the top of Booster Tower, you have to battle Knife Guy and Grate Guy.

Battle Knife Guy and Grate Guy

Focus all attacks on Knife Guy until he is defeated. If Geno and Bowser are in your party, have Geno use Geno Boost on Bowser, and Bowser can just use his regular attack with Chomp. Mario should use Super Jump on Knife Guy. Grate Guy can use Echofinder, which prevents a character from using Special attacks. Grate Guy can also inflict sleep on characters.

Eventually, Knife Guy will stand on top of Grate Guy, and Grate Guy will start using Meteor Blast, which does high damage to all characters. Knife Guy can use Blizzard, which does a small amount of damage to all characters.

When Knife Guy is gone, Mario can just use regular attacks with Masher on Grate Guy.

Afterward, you have to run up Booster Hill. You can only jump, and move left and right. Don't get hit by the barrels or Snifits or you'll go farther back. Try to jump on them instead, to go farther forward. If you are close enough to reach the princess, she will give you Flowers if you make contact with her.

After the race is over, you arrive in Marrymore.