How do you beat Pokemon mansion in cinnabar island?

To beat the Pokémon Mansion on Cinnabar Island, explore the various rooms to find keys and items while battling wild Pokémon and Team Rocket members. Collect the three key items—namely the Secret Key, which unlocks the Cinnabar Island Gym. Be sure to read the various notes scattered throughout to learn about the mansion’s history and the creation of Mewtwo. After gathering the key, exit the mansion and head to the gym to challenge the leader, Blaine.