Boss Butch! is a sub-game similar in concept to boss-fighting endurance sub-games from Kirby games, but vastly updated. It's generally viewed as the hardest sub-game in Mario's Fun Pack. Before Mario can fight any enemies, he will be in a room where he can choose a power-up from a ? Block. Mario is allowed to take a Helper in at will, though the player should note that the prescene of a Helper on-screen gives the boss more health. It should also be noted that Buzzy Beetle is the only boss NOT to appear in Boss Butch! There is also a Warp Pipe in the room. When Mario goes in it, he will go to fight any of the following bosses in random order:
5. Lemmy Koopa
6. Bowser Jr.
10. Iggy Koopa
11. Larry Koopa
12. Twin Reznors
14. Ludwig von Koopa
15. Doomship Reactor
16. Boom Boom
18. Bowser is always fought last in this mode.
In between enemy fights, Mario will be in a room where there are two randomly-generated power-ups, along with a limited number of hearts that will not respawn after being grabbed. If Mario gets a heart in the room between fights, a half heart (which restores less health and will not be replaced or respawn) will replace it the next time he visits the room. However, only 13 bosses are needed to complete this sub-game.
If Boss Butch! is cleared, the Nintendo Jukebox will be available for listening.