Let's keep this whole Halloween themed submissions thing going, shall we?
I already played through Luigi's Mansion: Dark Moon and my favorite boss was the Harsh Possessor. Why? There were so many factors that just made this so fun for me.
1) the path to the boss was a haunted staircase where choosing the right path lets you advance while a wrong choice sends you back to bottom and possibly another ghost to deal with
2) the music builds in both the boss battle and the staircase sets the mood
3) Unlike the other Possesors who have a convoluted gimmick you have to disable, the Harsh Possesor's gimmick is significantly easier to get around
I took some creative liberties as I thought most of ghosts in the game are far too cartoony for my tastes.
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