It could be an unknown elder ninja. This elder has seen the rise of fanfiction and its potential threats to the real ninja world. He has unique powers and techniques that are specifically designed to deal with such non - traditional threats. So, he takes Naruto under his wing and trains him to kill the fanfiction.
Maybe Kakashi could be the one. He's always been Naruto's mentor in many ways. He has the knowledge and skills to train Naruto for such an unusual task. He could use his Sharingan to analyze the fanfiction and teach Naruto how to spot the dangerous elements within it.
Another option could be Itachi. Although Itachi is from a different village and has his own complex situation, in a fanfiction scenario, he could secretly train Naruto. Itachi has extremely powerful jutsus and a deep understanding of the Sharingan. Training under him would give Naruto a very different perspective on power and combat, not to mention some insights into the Uchiha clan.
It could be a rogue ninja from a forgotten clan. This ninja might have a vendetta against Naruto's village and has powers that are difficult to counter. So, Naruto has to train to kill him to protect everyone.
Well, Kurama might train Naruto by teaching him how to better control and access chakra. Since Kurama has a vast amount of chakra, it could show Naruto unique ways to harness and use it. For example, it could guide Naruto in creating more powerful and complex jutsu by using their combined chakra in a more efficient way.
He might kill an enemy shinobi from another village. For instance, if there is a particularly aggressive and cruel shinobi from the Sand Village during the Chunin Exams who poses a great threat to Naruto's friends or the Leaf Village in general, Naruto could be forced to kill him in self - defense or to protect others.
Well, it could be about Naruto being forced into a dark situation where he has to train for killing. Maybe he's been misled or there's a great threat that makes him think this is the only way. For example, he might be tricked by an enemy who disguises as a friend and convinces him that the only way to protect the village is to become a killer.
Well, perhaps Naruto is forced into a strange situation where he has to combat the ideas and stories in fanfiction that are causing some sort of chaos in the ninja world. Maybe some evil force is using fanfiction to distort the real story of the ninja world and Naruto is the only one who can stop it by 'killing' these false narratives.
Maybe it could be Godric Gryffindor. Since Harry is in Gryffindor house, Gryffindor himself might take it upon himself to train Harry, passing on his courage - based magic and sword - fighting skills.
Another aspect of the training could be mental discipline. Hayate may introduce Naruto to meditation - like techniques to help him focus his chakra more effectively. He'd also likely teach Naruto how to keep a calm head in the heat of battle, sharing his own experiences to illustrate the importance of mental fortitude.
His father, Minato, could be a likely candidate in a really dark fanfiction. Maybe he was resurrected in an imperfect way and his mind was warped, causing him to turn on Naruto.
Maybe Sasuke. Sasuke is extremely proficient in kenjutsu. In fanfiction, he could see Naruto's potential and decide to train him, sharing his knowledge of sword fighting with Naruto. This could also be a way for them to bond and grow stronger together.