I don't remember if it's explained in the manga and people are just being forgetful, but the reason why benet could die from a "stab wound" was that Ruphas was doing enough damage that her regeneration became useless.
It looked like they were talking in real time where benet could easily heal the shit out of that wound, but their Agility stat is so high that when those 2 concentrate their mind, time starts to slow down in their perception, to an absurd degree.
To make it simple, say, the whole fight happened in a second in real time. Her regen just couldn't keep up lol