Follow Mario from Hellen Gravely’s office to the rooftop when the collection is ready. King Boo’s fight has three phases, ending with three copies and a four-minute countdown.
Before the point of no return
Finish missing gems, Boos, floor achievements and money collection before following Mario into the final sequence. The completed ending does not create an explorable post-game hotel; loading an earlier checkpoint lets you revisit the collection.
Avoid the attack patterns
Move off the marked floor before lightning strikes. Burst over low tongue sweeps and shockwaves, and pass beneath raised tongues when there is a clear gap. Keep moving between fireballs. During the repeated stomp attack, time each Burst to the landing shockwave.
Bomb opening
After a spiked ball breaks, vacuum a live bomb and shoot it into the real King Boo’s open mouth. Move in while he is stunned. Use Luigi and Gooigi together on his tongue, pull away until the slam is ready and slam him onto the roof.
Two copies
The second phase adds a decoy. Identify the real King Boo by his four visible teeth; the copy only has two fangs. Remember his position rather than choosing whichever copy is nearest Luigi. Dodge the combined attacks, wait for another bomb opening and use both characters on the correct tongue.
Three copies and four minutes
The final phase has three Kings and a visible countdown. Identify the real one, avoid the extended attack combinations and keep a bomb ready only when his mouth is open. A bomb thrown into a closed mouth or at a decoy wastes the opening. Land the final two-character tongue slam before the timer expires.
Ending rank
The rebuilt hotel receives rank A, B or C based on the money remaining when the game ends. Rank A requires at least 70,000G; rank B begins at 30,000G. There is no rank S in Luigi’s Mansion 3. Gems and Boos have their own completion rewards and do not directly set the ending rank.