Well, he was originally supposed to fight someone else who was weaker. So him dealing damage to the strongest G4 is a feat in of itself. Plus I bet Kou loved him coming back from the brink of defeat to inflict that much damage.
This whole manga premises is so souls like youre basically one of the undead the more you die the more you become hollow the end goal is to get back to the living world which is very similar to bloodborne or elden ring