Good, manga clearly showed that there was noone anywhere near the vicinity of him, aside from the girl covering behind him.
So unless it was caused by reactive armor, the cowering girl, or a ranged attack, we know for sure he is super stronk and playing stupid. Cause it's physically impossible someone interjected without showing up in frame, and mangakas would never artificially create cliffhangers by intentionally drawing things incorrectly so ppl have to wonder what happened until the next chapter, only to reveal it was something without any hints in the prior chapter. Absolutely never. Impossible for one to even think of the idea. Like, what kind of idiot would have a noble lay hands on the mc only to have the arm lopped off forcefully instantly, and next chapter show it was a dragon randomly flying past without any foreshadowing or visual clues or cues?
Additionally, we can eliminate the cowering girl, as she is cowering + not shown to be psychic and able to telekinesis stuff with closed eyes. Same for reactive armors as I don't remember any mention of those. So either ranged attack, or mc is stronk af.
/s