going from the game's mechanics, there are basically two things ever that dante could not royalguard, and both of them were essentially a health drain baked into the level to encourage you to hurry. Blocking two simultaneous attacks is easy, as one block will essentially make you invulnerable for that frame, and if the attacks aren't simultaneous, a successful first block would reset the cooldown on a subsequent block to zero.
Basically, there is no attack in the history of recorded DMC that could punch through a royalguard, and blocking 1, 2, or 100 supermen would be just as trivial. The style is so overpowered that it is never mentioned in any cutscene, lore, or backstory, because simply bringing it into the same realm as cutscene dante would reduce every single fight down to "why didn't he just royal guard that? well he can royal guard that, he'll be fine"
And then after he's blocked those punches, he can reflect the absorbed damage back on his attacker. I think the real question is: can superman stand up to several of his own punches?
Naturally there's more to consider here. Is dante fast enough to accurately block an indefinite number of punches from even a single superman? Even with quicksilver, that's questionable. But can he soak that punch, assuming he landed the block? Absolutely.
TL;DR - royal guard has never been beaten by any enemy attack, ever. For this to happen would literally be to rewrite everything we know about royal guard right now.
