How do you battel Cynthia in Pokemon HeartGold?

1. Defeat the Elite Four. Cynthia is the Champion of the Sinnoh region, and you must defeat the Elite Four before you can challenge her. The Elite Four are located in the Pokémon League building in Victory Road.

2. Enter the Pokémon League Champion Room. Once you have defeated the Elite Four, you will be able to enter the Pokémon League Champion Room. This is where you will battle Cynthia.

3. Choose your team of Pokémon. You can bring up to six Pokémon with you into battle against Cynthia. Make sure your team is strong and well-prepared, as Cynthia is a tough opponent. Some Pokémon to help you could be:

- Ice-Type Pokemon: Froslass, Weavile

- Fire-Type: Infernape, Houndoom

- Electric-Type: Luxray

- Fighting-Type: Gallade, Lucario

- Psychic-Types: Alakazam, Starmie

4. Battle Cynthia. Cynthia uses a variety of powerful Pokémon, including Garchomp, Lucario, Milotic, Roserade, and Spiritomb. Be prepared for a tough battle, and don't give up!

5. Defeat Cynthia. Once you have defeated Cynthia, you will become the Champion of the Sinnoh region! Congratulations on your victory!