Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
In this 'Harry Potter Trained by N Fanfiction', perhaps Harry is trained in the art of magical combat in a more intense way. He might be sent to a secret location where he has to endure rigorous training. This training could involve facing powerful magical creatures, which helps him build his confidence and magical strength.
It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.
I think it could be Neville Longbottom. Neville has grown into a strong and confident wizard. He could have discovered boxing as a form of self - defense and then decided to teach Harry. Neville would bring his own unique perspective, having been bullied in the past. He'd train Harry to be both strong and fair in the boxing ring, emphasizing the importance of honor and respect in combat.
Maybe it's a member of a long - lost ninja clan. This clan member could have some special skills and knowledge that they want to pass on to Naruto. They choose to train him in secret to protect their clan's secrets from falling into the wrong hands.
It could be a deranged fan who has found a way to bring their dark fanfiction ideas to life. This person might be jealous of the original story and wants to create chaos in the wizarding world through their fictional concepts made real.
She could be an ancient Celtic goddess, like Brigid. Brigid is associated with magic, healing, and smithcraft in Celtic mythology. In the fanfiction, she might see Harry's potential and decide to train him in her unique magical arts.
Sirius Black could be a trainer. Sirius, being Harry's godfather, would be highly motivated to train Harry. He could use the time - turner to go back to a time when he was at his peak in terms of combat skills and pass on his knowledge to Harry, especially in the areas of dark - wizard fighting and survival skills.
There's a possibility that his trainer is a self - taught expert. This person could have learned about pistols through years of exploration and practice. They see potential in Harry and take it upon themselves to train him, sharing their unique and unorthodox methods of pistol training with Harry.
It could be Zeus. He is a powerful god and might see Harry as a worthy candidate for training. He could teach Harry about leadership and power control.