How to use pokeblue_debug.gbc debugging features
On the title screen, press Select to access the debug menu, which has two options:
Selecting the FIGHT option prompts you to nickname your Rhydon, then takes you to a battle with a wild Rhydon:
Rhydon is the only mon in your party. Whatever move you select, Rhydon will instead use Pound. Your Item menu is not intitialized and is filled with garbage data. Ending the battle (win, lose, or run) starts the cycle over again. You must reset the game to leave the battle loop.
Many of the parameters of the debug battle can be modified in debug_menu.asm.
Selecting the DEBUG option prompts you to nickname your party, then puts you into the end of Oak's introduction speech with the default name NINTEN. You begin outside your house, not in your bedroom.
You begin with a full party that knows most field moves (Cut, Fly, Surf, Strength, and Dig), a complete Pokedex, all Fly locations available, and 7 badges. All of this can be edited in debug_party.asm.
While walking in the overworld, hold B to prevent trainer and wild battles. Some scripted battles cannot be avoided in this way (e.g. vs. Rival in Cerulean City).