@Shakur
He assumed too much based on his own logic. You also did the same.
What if a weapon from their world doesn't work on that monster? What if the metal is weak against that monster's hide, fur, whatever? What if this hide, fur, whatever gave it a layer of dermal armor that no weapon in their world could pierce without utilizing magic? Also, as you said, what if she's not skilled enough to use a weapon? What if her skill primarily lies in using her magic, filtering that through her body and using her body as a weapon? There are so many unanswered questions that the MC didn't think about before he opened his mouth and assumed he knew better.
You're right, a person doesn't have to be an expert at fighting to know that using different weapons in different situations may be beneficial, but said person has to have some relative knowledge of said weapons. For instance, with my far less than beginner knowledge
(essentially no-knowledge) about swords, I might be able to get away with only telling someone obvious things like what end they probably should be holding, or maybe to watch how they swing it, maybe surmise that a heavy sword may not be good for someone without a decent amount of upper body strength, etc. Some of those things are deduced through observation, but as some of them get into more advanced things, they have a higher risk of not being right without getting more information about swords and sword fighting techniques. If I assume I know better than the person using the sword because I'm an expert at management in a corporate company then I'm going to end up putting my foot in my mouth beyond obvious observational things that have a high likelihood to be correct. And with limited to no knowledge about swords, the number of those obvious things I would get right drop significantly.
That's my overall point. He has no knowledge of how magic worked in the new world beyond observing it a few times. We're supposed to believe that from observing it a few times that he knew enough to critique her on what she was doing when she is obviously the warrior and he is the manager and diplomat. But like I said, the author will forcefully make him right when the two reconcile.