How do you get the key to Luigi's door in super Mario 64 d's?

All you have to do is get nine stars and go into the room behind the rec. room then you jump in the painting of Mario and find goomboss. Then destroy goomboss by eating his minions and spitting them on his back. Then after three lives he will explode and the key will come flying out. p.s. I don't know how it got in his body!