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 starts a battle with a wild Rhydon:
Whatever move you select, Rhydon will instead use Pound. Rhydon is the only Pokémon in your party. 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 members, then brings up the end of Oak's introduction speech. You begin the game outside your house, not in your bedroom.
You start out with a full party that knows most field moves (Cut, Fly, Surf, Strength, and Dig), a complete Pokédex, 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 wild Pokémon encounters and prevent NPC trainers from seeing you. You can still initiate a trainer battle while holding B by talking to the trainer. Some scripted battles cannot be avoided in this way (e.g. vs. Rival in Cerulean City).