Get the Golden Key behind the cage puzzle 6F Castle Mac Frights Luigi's Mansion 3
Use Gooigi to pull the rope to open cage - Luigi can then walk into the cage to get the key. Gooigi can't because there is water and he dissolves in water. There will be ghosts - Luigi is in the cage and will be spiked by the moving walls unless Gooigi vacuums all the ghosts.