@Sourish Saha
Think about it this way, He's not really struggling against this guy, but the power this guy got from "Davin", so really he's struggling against the final boss.
Also don't forget, as shi*** as his personality and goals may be, this guy is still a long established elder of one of the 3 strongest martial clans, so he's bound to be at least pretty strong, plus having received the demon power, and the MC having to fight him in a complicated situation.
