This was skipped in the manga, but the highlight weren't actually simply killing that Charybdis copy by raw power alone. This Charybdis copy still posses that magic nullification skill (or whatever the name, I forgot) and regeneration as broken as the original, even though the total energy amount or HP is less.
Benimaru, by manipulating his skills in a certain way, were able to burn that fake Charybdis, ultimately bypassing its magic nullification and regeneration skill using only his proficiency in it. A feat which Alvis said only available to being as strong as normal demon lord or above, like her master Karion. Effectively putting Benimaru power-level on that spot.