Start up a new game and go to Field, Defeat Qixchu then go to your "Box", Press select until the box becomes glitched, then leave and press "Next". You will fight Qixchu again and have a huge amount of health. The amount of times you have to press select varies (between 4-65+) depending on what item you got (no item is also an option) and how many times you died during the first level (0, 1 or 2).

The lowest amount of selects so far has been 4.

WARNING: doing this glitch deletes ALL your progress in return for giving you the health, use it wisely.