UFC champ Amanda Nunes says it would be "impossible" for Claressa Shields to beat her in an MMA fight ... and she's warning the boxing star to back down before she gets hurt.
Shields is the biggest star in women's boxing by far -- she's undefeated, holds multiple belts and Olympic gold medals. But, she REALLY wants to fight Amanda and recently started MMA training in hopes of making it happen.
The bad news for Shields ... Amanda says she would SQUASH Claressa like a bug.