What is KOS-MOS doing on the cover?
Also, yes, when fighting Superman holds back because he doesn’t want to kill anyone, but just because he holds back doesn’t make him any less invulnerable (kryptonite or bad writing might, though). Similar logic should apply, especially given that as a demon lord you don’t have to let your enemies live. Also, magic should totally be able to strengthen and protect him from his small-fries enemies.
Momonga (or Ainz, either works) was similar in that he preferred to hold back, but he was still quite overwhelming.
Then again, I don’t really see him struggling against anyone, so, meh.