Related Clues:

• Archenemies, maybe
• Formidable opponents
• Superheroes, to villains
• Agents of retribution
• Banes
• Plan foilers, perhaps
• Ruins
• They're unconquerable
• Batman and Robin, to Catwoman
• Foes
• Fierce foes
• Formidable rivals
• Unbeatable archenemies
• Holmes and Moriarty, e.g.
• Usually unbeatable rivals