I believe this is Nito's Lazy Foreign World Syndrome.
From Baka-Updates:
Hidako Masamune is a high school student who suffers from continuous bullying by his classmates. Becoming tired of it all, he decides to commit suicide, but instead he is transported to another world along with all his classmates. When they arrive, they find that they now possess amazing abilities but he was only granted the status of a "healer," the weakest skill of all. The king decides to get rid of him by transporting him to another far away place, but not before Hidako swears revenge against the king, the kingdom, and all of his classmates.
In chapter 9, the protagonist, Hidaka Masamune, and his companions -- a knight named Sierra Ecarlat, and a paladin named Toatrica Rosev Urzola -- are looking at bounty notices in an adventurer's guild, when they see one for a 'Kelaine,' a large beast with two horns that lives in the forest.
They head to the forest to dispatch it, but when they get there, they discover that it's actually a 'Noot Kelaine,' a more powerful version rated as a grade A monster. They also encounter another party of adventurers in the forest, who've already had a run-in with this thing and are in bad shape. Sierra tells those other people to get out of there, while she and her comrades deal with it.
Sierra has the first crack at the Noot Kelaine, but finds that her magical attacks have no effect on it, as it can control magic particles with its antlers. Toatrica's magic also has no effect. Hidaka reasons that if they destroy the antlers, it won't be able to block magical attacks anymore, so he teleports above it, lands on its head, and snaps the antlers off with his bare hands. He then kills it with a magical attack called [Goddess' Bloody Tears], but asks Sierra to keep what he did a secret, as he wants to maintain a low profile.
Moments after killing the Noot Kelaine, the group hears the sound of people approaching on horseback, and Hidaka decides to leave with Toatrica, to avoid being discovered. He tells Sierra that he trusts her to explain what happened without mentioning him, and that they'll meet up again later in the city.
The people who arrive shortly thereafter are the leader of the king's knights, Reinhart Rickman, and two more knights, Reyd Black and Hilda Ecarlat. They hurried here upon hearing reports of the battle, but arrive to find that all that's left of the Noot Kelaine is the unidentifiable mess of its remains. Reinhart asks Sierra what happened, and what happened to the other two adventurers who were reported to have been involved, but she just tells him that they left, and remains otherwise tight-lipped. He reasons that Sierra couldn't have killed the Noot Kelaine alone, as it'd take at least four knights to defeat one, so the other two adventurers must've been responsible.
Sierra later reunites with Hidaka and Toatrica in the city, but she was followed by one of the knights, Reyd, who wants to know who took 'his' prey, by killing the Noot Kelaine, and has now deduced that it must've been either Hidaka and Toatrica. Toatrica claims she was the one who killed it, and Reyd states that he does feel amazing magic power from her, whereas he doesn't feel any from Hidaka. He then begins attacking Toatrica with a series of melee strikes and some fire magic.
Reinhart shows up and tries to stop the fight, telling Reyd that killing the Noot Kelaine doesn't make Toatrica an enemy, and that she was simply exterminating a monster as per the bounty request. But Reyd argues that she must have something to hide, since she ran away, and that someone that powerful is a threat who should be disposed of before they get any stronger. He then renews his assault on Toatrica, which is when Hidaka steps in and knocks him out with one downward punch that leaves a huge crater in the ground.
Seeing that, Reinhart realises that Hidaka is the one who killed the Noot Kelaine, and instinctively draws his sword. After Sierra asks him to put his sword away, he regains his composure and apologises, explaining that he was a bit startled. He also explains that everyone is born with magic power, and that even if Hidaka's was extremely weak, Reyd still should've been able to feel it. However, if Hidaka's power far exceeded his and Reyd's, that would explain why neither of them could sense it. He apologises again for Reyd's actions, and says that he'll take full responsibility.
