Again with the power levels. If this guy killed 100 exorcists, including Commanders, and they get stronger by killing them, shoouldn't this guy be in a level where he needs alll the exorcists to kill him? Hence the group is dead for sure and cannot even scratch him. Also, if he is on a similar level by Hijirimaru if we take his words as true, and he was almost killed by Simon, this doesn't make much sense.
There is also the fact that if the impurities get the exorcist's powers, from where each generation gets them? They would be out of guardians afer 1000 years.