You can't defeat him? Then that is surely, because he is the demon lord.
Yeaaa...I'm also wondering, why the author suddenly did hint, that MC is not battle hardened. All I can imagine is, that while he fought monsters secretly, so that he isn't caught being the hero, he might have avoided fighting human foes. But then again, he must have faced humanoid enemies, that would just train him in this way, see Sir Goblin.