Welcome to King Boo's Illusion, the final battle in the game. Now this battle takes place across three segments, with all of them except for the third consisting of two parts.
In the first part of each segment, you'll battle King Boo directly. And the first thing he'll do in these segments is summon 3 spikes balls, which will roll in the direction that the arena tilts, which is dependent on where you stand. And dodging these balls is actually tougher than dodging King Boo himself, who will follow you at a slow pace.
However, King Boo will occasionally pound the ground, launching the balls into the air--this is your chance to attack! You'll have to lure the King into the shadow of any one of the balls so that it lands directly onto him, making him vulnerable to being captured. Now actually luring King Boo into the shadow can be a little tricky. You'll want to position yourself so that the shadow is between you and the King, then once he crosses into it, shine your flashlight onto him to hold him place and make him vulnerable. If it's a direct hit, he'll be squashed, which is your chance to go in for the capture. And if he tries to escape your, you can pull him back down with a Charged vacuum attack.
Once you fully deplete his health, you'll enter the next segment where you have to run away from an oversized King Boo. There aren’t really many tips I can give since it's all about quick reactions, but when you reach the pair of staircases, take the one farther back to avoid an escalator going the wrong way.
After the running segment, you'll get to take on King Boo for a second time. But this time, he'll occasionally shock tiles on the ground, turning them into either sandy or icy terrain, making it difficult to move around. Obvious you should try and avoid these, especially during the actual shocking process. Otherwise, the battle is the exact same as before, only now one of the Spiked balls is much larger.
Afterward, you'll have another running segment to deal with before it's onto the final arena battle with King Boo.This time you'll have to avoid an occasional massive lightning attack, so make sure to run to the one tile that's not lit up. Otherwise, the strategy is still the same. Once you take him down here, the final piece of the Dark Moon is yours for the taking,