Instructions
Jump to avoid Bowser's fireball attacks in the first phase of the fight. He will shoot a fireball out when he takes a step toward you, and then another when he steps back a few spaces.
Run underneath Bowser when he leaps high into the air. Hit the red switch on the floor to cause the floor to collapse, initiating the second phase of the fight.
When Bowser reappears, head right and use Bowser's fireball attacks to clear a path through the blocks, jumping to avoid the blast just as it lands onto the blocks.
Continue heading right, staying high to collect the mushroom and Fire Flower boxes. Ride the platform to the bottom of the stage, and collect the "Propeller Mushroom."
Continue heading right, jumping on the floating platforms to avoid the lava waves that Bowser creates.
Jump up to the final platform, and hit the large red switch on the ground. This will end the fight and save the Princess.