Possible Clues:

• Formidable foes
• Harry Potter and Lord Voldemort, e.g
• Sinestro and Professor Zoom, in their respective universes
• They can't be beaten
• Superheroes always have them
• They're usually unbeatable
• You can't beat them
• Holmes and Moriarty, e.g.
• Luthor and Brainiac, to Superman
• Pokémon's Gary and Ash, e.g