Boolossus

Boolossus is the third Boss Ghost in Luigi's Mansion. He worked alongside King Boo before his imprisonment. After being freed from his portrait, Boolossus split into 15 normal sized Boos hid in the Storage Room with all of the other Boos. Upon his release, Boolossus fled to the Balcony. After collecting a certain amount of Boos, Luigi will gain access to the Balcony.

Capture
On the Balcony, Luigi's Gameboy Horror will detect a large amount of Boos in the area. The 15 Boos will form together into Boolossus. The Unicorn Statues freeze and will spawn Ice Element Ghosts. Luigi must use the Poltergust 3000 to drag Boolossus into one of the Unicorn Statues. The horn on their heads can be used to puncture Boolossus splitting him into 15 smaller Boos. Luigi must capture an Ice Element Ghost and freeze the Boos. The frozen Boos can then be vacuumed into the Poltergust 3000. This process will be repeated until all of the Boos are captured. As the number of Boos decreases, Boolossus will shrink but the Boos will flee when Ice is expelled from the Poltergust 3000. The Boos may need to be brought closer before attacking with Ice.

Pal Hidden Mansion Capture
In the PAL Hidden Mansion, Luigi will ride on the Poltergust 3000 during this fight in attempt to capture Boolossus.