This is otherwise known as the "Danger Mario" setup. By reducing your maximum HP to 5, you can make it so certain badges activate all of the time. This makes most attacks do maximum damage making the game very easy!
Here I show how to create this setup, then I test it out on some of the hardest bosses in the game. The "super secret boss" that unlocks once you have done most other parts of the game can be found in the pinned video.
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Timestamps
00:00 Introduction
00:39 Setting This Up
04:28 Don't Do This!
05:17 Boss Testing
09:20 Is This Cheating?
Enjoy!