The The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 4 (light novel) — Rui Tsukiyo

Testing into Class S has given Lugh the perfect opportunity to get closer to his target, the hero Epona -- assuming he can survive being the hero's personal punching bag! It will take all of Lugh's skills to look past his quarry's monstrous strength and discover an exploitable weakness, but further complications arise as he's secretly tasked…with protecting Epona from a potential assassin?!