Thanks for the chapter, NGS.
lol MC-kun here can take so much abuse--he'll make for the perfect salaryman! Such a heroic trait (and you know it is, because he can make scaaary faces at the end).
Just don't give him any sealed jars to open.
That said, it went as I thought it would: the author fed MC-kun the victory. This guy Arai was hyped up to have been someone that even Karin couldn't reliably beat, never mind Touka and the girl with the weapons--and all three are possessed of combat experience. If MC-kun here could beat Arai at all, then there's no way there should be the distance between MC-kun and Karin that the story's been establishing--that is to say, Karin isn't anywhere near as good as the story's established her to be, because she's hardly better than someone with little to no fighting experience.
Speaking of fighting experience, which Arai has to have--and enough of it, paired with masculine strength, to give Karin serious trouble: MC-kun is clearly displaying he's a one-trick pony in terms of offense; for the most part, he kept low-kicking Arai in one place. If logic doesn't tell you to avoid having one spot on your body tenderized, you should at least get used to the extremely simple attack pattern you're facing. If not that, at least pay attention to the increasing pain in the spot that keeps taking hits. To reiterate: if this is how Arai fights--if he's this simple-minded--then Karin (who obviously has to rely on technique more than a male would) should have laid him flat with ease, in the past; Arai was/is not strong enough to negate technical superiority, either. All of this was just the author giving Can-can Boy the win by railroading Arai into eating every instance of the former's singular offensive tactic.
Well--at least MC-kun didn't wax emotional over Arai in the end. I could easily see it happening.