It would also depend on the nature of the hero. If we consider someone like Makeda or Chroma or Allure, they are fairly powerful combatants and would likely be able to hold their ground against a Realta or two. But if we consider someone like Gerrik (ignoring his divine equipment) or Susa, who weren't designed as combatants, then they would struggle in such a fight.

P.S. Although I agree that none of the heroes currently available are capable of taking down all the Realta.