Kruller

Kruller is the third boss encountered in Luigi's Mansion 3. He is encountered and fought on Floor 3. Defeating Kruller rewards Luigi with the elevator button to the second floor.

Appearance
Luigi briefly witnesses Kruller as the latter is doing patrols on the third floor. After hearing noises, Kruller panics and tries to investigate the commotion, only to discover a mouse inside a flash machine. Dismissing it, he then enters his office, although he accidentally drops a key (the first key to a puzzle that results in getting the key to his office) when entering it. When Luigi enters the office afterward, he briefly faints from shock at Luigi entering, before entering the next room to find a suitable weapon to defend himself (humorously getting stuck on his back mid-roll), ultimately getting a water pistol.

Battle
Because Kruller is behind a barricade, the player has to fight him as Gooigi. Gooigi has to flash at Krueller and then suck in his glasses before flashing at him again to stun him. Kruller will also shoot squirts of water at Gooigi; if any of them hit him, he will dissolve, wasting time. On the final round, he'll make it more difficult to suck in his glasses. Upon being defeated, Kruller will struggle to avoid being sucked in to no avail. Gooigi then retrieves the Floor 2 button, with it being humorously revealed that Luigi slept through the entire battle. After defeating Kruller in 2 player mode, Luigi, who was watching the battle from outside, takes all the credit saying that he did it, after which Gooigi humorously copies Luigi as he had actually defeated Kruller.

Trivia
Kruller is one of the few bosses in a Super Mario game, besides tutorial bosses in the Mario & Luigi series, where you are guaranteed to win; you must fight him as Gooigi, and if Gooigi is hit by his water pistol, Luigi can simply spawn him again.