Hit King Boo with a falling spiked ball
Keep King Boo under the shadow of a falling spiked ball. Shining the light at him holds him in place long enough for the ball to strike his crown; do not remain beneath the same shadow yourself. Dodge the other rolling balls, then vacuum King Boo while he is stunned. Pull against his movement and complete the capture phase. His first two defeats lead to chase sequences rather than the end of the battle.
In the hallways, keep running and react to the furniture, armor, sand and ice ahead. Choose the real stair route instead of charging into a false one. Avoid stopping to search rooms during these escape sections. Back in the arena, repeat the shadow-and-crown setup while accounting for the changing floor.
Survive the last arena and rescue Mario
The later arena adds hazards underfoot, so secure your own footing before positioning King Boo. During the final lightning pattern, move to the safe tile before returning to the falling-ball setup. After the third successful capture phase, vacuum King Boo for good. Use Dark-Light on Mario’s painting to release him and complete the story.
For full completion afterward, return to the mission selector for missing gems, Boos and three-star ranks. The final E. Gadd medal also requires completing the Vault, including the ScareScraper ghost collection. Finishing the credits alone does not fill those optional records.