Well, in some fanfictions, Harry might draw on his own experiences facing dark magic. He could use his adventures with Voldemort as real - life examples to make the lessons more vivid. For instance, when teaching about defensive spells against the Imperius Curse, he could talk about how he resisted it in his encounters.
Maybe a wizard who has traveled to far - off lands where voodoo is practiced could teach Harry. This wizard could have seen the power of voodoo and thought it could be useful in the fight against the Dark Arts in the wizarding world of Harry Potter.
It could be Hagrid. He's always had a connection to strange and powerful things. Maybe he stumbles upon the knowledge of nen during one of his trips in the magical forest and then passes it on to Harry.
Nick could teach Harry Potter through storytelling. Since he has been around for a long time, he must have countless tales to tell. He could use these stories to teach Harry about magic, its history, and the values of the wizarding world.
Dumbledore could be the one to teach Harry about Haki. Dumbledore, with his vast knowledge of magic and the unknown, might have come across Haki in his travels. He could recognize the signs of Haki in Harry and take it upon himself to guide Harry in understanding and controlling this new power. He would use his calm and wise approach, just like when he teaches Harry about other aspects of magic.
Snape teaches Harry about potions in a rather strict and demanding way. He often criticizes Harry's mistakes harshly but also imparts important knowledge about potion - making techniques, ingredient properties, and the correct procedures to follow in the classroom.
It could be Molly Weasley. She raised a large family of wizards and witches, so she has a lot of practical experience. She would be able to teach about things like cooking magical meals for the family, dealing with sibling rivalries among magical children, and how to keep the family safe during dangerous times in the wizarding world.
Maybe it could be an ancient magical being like a centaur. Centaurs are known for their knowledge of the stars and ancient magic. They might take Harry under their wing and teach him old magic that has been passed down through their generations.
The goblins could teach Harry some defensive magic that is unique to their kind. This could be magic that is not taught in Hogwarts. For example, a type of shielding spell that is more effective against dark magic that specifically targets goblin - made items. This would give Harry an edge in battles against the dark forces.
Snape could teach Harry about the importance of potions. Potions are a crucial part of magic in the wizarding world, and Snape is an expert in this area. He could pass on his in - depth knowledge of potion - making techniques and the properties of different ingredients.