This manga adaptation sure f'ed up this part of the story. It's all over the place. The novel was much more straightforward in delivery, no chalked-up excessive drama, and clearly showed how OP MC is (which is the main reason why these fools keep going down that realm to get him back to the human realm). This manga adds too much garbage dramatization that doesn't even increase tension in the fight.

In fact, in the novel this fight was way more simple, MC overwhelming this foolish brother to the point it clearly shows how far the gap between them is. MC had more trouble dealing with the fact that he had to draw out info from the other humans with the brother, and keeping them alive while doing so as the brother goes berserk on a killing spree. MC's entrance was also different, him showing his swordsmanship skills outright, which establishes his level being way above skilled knights that were with his brother.