Instructions
Use the vacuum to grab one of the spiked balls that Bowser shoots from his mouth. The balls are too heavy for the vacuum to bring in, so walk toward one to secure it.
Wait for Bowser to bend forward and open his mouth. Walk backward to avoid his fire breath, then hit him with the spiked ball. Bowser's head will come off, and King Terasa will emerge.
Avoid the ice blast from Bowser's head as you use the vacuum on King Terasa. Once you've depleted some of King Terasa's hit points, he will climb inside Bowser once again.
Repeat the process of hitting Bowser with the spiked balls, then vacuuming away King Terasa's hit points. Eventually, Bowser's head will attach itself backward. He will then stomp around the roof trying to fix it. Take cover behind the chimneys to avoid suffering damage. Continue the fight once Bowser is back to normal. When King Terasa's hit points are gone, Luigi will collect him inside the vacuum and Bowser will fall.