I think it's mostly a misinterpretation of how the mc fights or people are assuming that all mc should be strong in the same ham-fisted way. His way is more technical. He finds the enemies weakness then stabs it with precision. Given the way he works, he couldn't be allowed that "one hit" like Shyning was asking for. His pattern is that he only chooses the "one hit" that completely devastates his enemies and nothing short. That's the style of swordsmanship he's being taught, no flashy exchanges or random swings, just a single perfect blow.
Still, I do think he needs to learn a bit more from the girls, particularly, he needs to learn from Fear. That physical reinforcement magic angels and fighters use would be a big boost for him, making that one opportune blow much more effective and defending him while he looks for the opening.