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.
It's possible that Salazar Slytherin trains Harry, although that would be quite a twist. Slytherin could see potential in Harry that others in the past didn't, and train him in some powerful and perhaps more morally ambiguous magic, which would create an interesting internal conflict for Harry.
One possibility could be that Harry goes back in time and is trained by some of the greatest wizards of the past. Maybe he learns advanced magic spells that have been long forgotten in his own time. This training could help him become even more powerful and perhaps change the course of future events for the better in the wizarding world.
It could be Dumbledore. He's always been like a mentor to Harry, so it would make sense for him to be the one to give an important talk, whether it's about Harry's destiny or how to handle the challenges in the wizarding world.
Dumbledore, even if not in person, could have left behind some clues or resources that help Harry. Also, the Order of the Phoenix members who are still around and believe in Harry's cause. They have their own skills and connections that could prove useful. For example, Moody could teach Harry some advanced defensive magic techniques to use against the Ministry's security measures.
The Death Eaters are an obvious choice. They've always been after Harry, so they could easily be the ones to kidnap him. They'd want to take him to Voldemort to prove their loyalty and also to finally get rid of Harry for good.
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.
Lupin could be a likely candidate. Given his experience and his role as a sort of mentor figure to Harry, he could give Harry the talk. It might involve Lupin sharing his own experiences with being a werewolf and how to deal with secrets and prejudices in the wizarding world.
Hermione would be a likely supporter. She is always loyal to Harry and if he presented her with evidence of Dumbledore's wrongdoings, she would probably side with him. She has the intelligence to help Harry strategize and find ways to expose Dumbledore. For example, she could use her research skills to dig deeper into Dumbledore's past.
Maybe Dumbledore, if he were still alive in the fanfiction. Dumbledore was highly respected and had a great influence. Knighting Harry would be a way for him to publicly recognize Harry's achievements and heroism on a grand scale.
His friend Ron could come back to help him. Ron is always loyal and might not want to leave Harry behind. He could bring his family's flying car again to pick up Harry.
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.