In fanfiction, Dumbledore could have set up a series of trials or quests for Harry before the final battle. These experiences make Harry stronger both magically and mentally. Dumbledore might also have a secret plan that he has been preparing for years. He could be using his own powerful magic in a covert way to weaken Voldemort over time, like gradually breaking down the protection spells around Voldemort's Horcruxes so that Harry can find and destroy them more easily.
Often in fanfiction, Dumbledore helps Harry understand the nature of Voldemort's soul - splitting into Horcruxes. He may guide Harry in the search for these Horcruxes. Dumbledore could also use his own powerful magic to shield Harry from Voldemort's attacks, at least until Harry is fully prepared for the final confrontation.
Maybe Harry uses some powerful and unexpected spells. For example, he could combine the Patronus Charm with a blasting spell. The Patronus might distract Voldemort while the blasting spell hits him hard.
Well, in some fanfictions, Harry might use some hidden magic he discovers in Azkaban. Maybe he finds a way to turn the Dementors' power against Voldemort. Since Dementors can suck out a person's soul, Harry could somehow direct them towards Voldemort, weakening him enough to deliver the final blow.
The relationship between them in fanfiction also affects their portrayals. If the fanfiction focuses on a timeline where Dumbledore and Voldemort were once friends or colleagues, their characters will be different from a story where they were always bitter enemies. Dumbledore might show more regret or a sense of failure if he couldn't prevent Voldemort from turning evil in such a scenario, and Voldemort might have a different level of respect or hatred for Dumbledore depending on their past interactions.
In some fanfictions, Harry and his brother might defeat Voldemort by discovering a new and powerful magic spell. For example, they could find a spell that nullifies the Dark Lord's most feared curses. This spell could be hidden in an ancient and forgotten magical text that they stumble upon during their adventures in the wizarding world.
Some fanfictions might explore a more nuanced past relationship between them. Maybe they had interactions in their younger days that shaped their future enmity. Dumbledore, with his deep understanding of magic and people, could have recognized the darkness growing in Voldemort early on and tried to guide him away from it, but Voldemort's pride and thirst for power made him reject any such guidance.
Dumbledore, as Harry's guardian in the fanfiction, would probably use his influence in the wizarding world. He could make sure that Harry is taught by the best teachers at Hogwarts. He'd also be there to offer advice and guidance, especially when Harry faces challenges like dealing with Voldemort or other dark forces. For example, he might teach Harry some advanced defensive magic privately to better protect himself.
The relationship would become very strained. Harry's rejection would likely make Dumbledore re - evaluate his actions and how he has interacted with Harry. Dumbledore might be shocked at first, since he probably thought Harry was on the same page as him most of the time.
As the story progresses, Dumbledore might try to make amends. He could start revealing some of his secrets to Harry in an attempt to regain his trust. Harry, on the other hand, would be hesitant at first but gradually start to listen and understand Dumbledore's reasons.
Well, it could be that Voldemort spoils Harry in fanfiction by taking away his normal life. Voldemort might use his power to isolate Harry from his friends at Hogwarts. He could put spells on the school or the people there to make them shun Harry. This would not only make Harry lonely but also disrupt his growth as a wizard and a person.