Wait until Bowser tosses a pair of MechaKoopas at you. Knock one out, then grab it and toss it up at Bowser. To do this, press X to pick it up, hold X and press Up, then release X. If timed correctly, it will hit Bowser in the head. After hitting Bowser twice, he will fly away. Small fireballs will fall from the top of the screen -- avoid them. When Bowser returns, the Princess will appear and toss either a mushroom or a flower at you. Bowser's floating thing will flip upside down, and a ball of blue will fall out and roll at you. Jump over it -- you cannot stand on it. He will eventually toss more MechaKoopas at you. Use the same method done earlier. After hitting him twice, Bowser will fly away again. More fireballs will fall from the top, the Princess will toss something at you, and Bowser will return again. This time, he will bounce around madly on the floor with his flying clown propeller. Do not touch the propeller. Eventually, he will stop and either flip upside down and drop a ball at you, or toss more MechaKoopas at you. Toss the MechaKoopas back at him. After hitting him three times, he will fly away and the Princess will fall out of the sky.