It's possible that the fanfic is set in a crossover universe. Harry might find himself in the world of the Daleks by some magical accident. He has to quickly adapt to the new and dangerous environment. The Daleks, with their cold and calculating nature, would see Harry as a threat. Harry, on the other hand, would be determined to protect this new world he's in, using his quick thinking and magical abilities to outwit the Daleks at every turn.
Perhaps in this fanfic, Harry teams up with some of his Hogwarts friends. They combine their magical knowledge and courage. For example, Hermione could use her research skills to find a weakness of the Daleks, while Ron provides support and loyalty in the fight. And Harry, being the main hero, directly engages with the Daleks using his powerful spells like Expelliarmus but modified to have an effect on the Daleks' technology.
Obviously, Harry would be one of the main characters as it's in the title. He's the one fighting the Daleks.
Perhaps in this 'harry fights back fanfic', Harry discovers some new powers or spells that he uses to fight back. He could be tired of being pushed around by Voldemort's followers or other bullies in the magical community, and he uses these newfound abilities to protect himself and his friends. For example, he might learn an ancient spell from a hidden book in the restricted section of the library. This spell gives him an edge in combat and he starts to turn the tables on his adversaries.
Well, it could be that Harry Potter discovers an evil empire within the wizarding world that has been secretly controlling things. He then uses his magic skills and the help of his friends to fight against it. Maybe there are new dark spells and powerful artifacts involved in this fight.
In this fanfic, perhaps the goblins have some sort of vendetta against a particular group that is also a threat to Harry. So they join forces. Harry could learn a great deal from the goblins' different approach to magic. And together they could stage some epic battles, using the goblins' underground tunnels as strategic points for their fight.
Well, it could be that Merlin, with his powerful magic from his own legend, sees Harry as a threat in Slytherin. Maybe he's been influenced by some dark force in the Slytherin common room. And so, he decides to fight Harry. Harry, of course, would use his Gryffindor - like courage and the spells he's learned at Hogwarts to defend himself.
One possible reason could be a misunderstanding. Maybe Harry is led to believe that Dumbledore has been hiding something crucial about his parents' death or Voldemort's return. For example, if Harry discovers some encrypted files in Dumbledore's office that seem to suggest Dumbledore had a prior deal with Voldemort, it could lead to a fight. Another aspect could be related to power. If Harry feels that Dumbledore is being too controlling over his actions and the use of his powers, he might lash out. In the wizarding world, power struggles are not uncommon, and Harry, as a powerful wizard in his own right, might want to assert his independence.
Perhaps in this fanfiction, the court in the wizarding world has been corrupted. Harry discovers some dark secrets within the court system. He then decides to fight against it to bring justice. He might start by gathering evidence against those who are misusing their power in the court.
Well, perhaps Teddy is in some sort of danger. Maybe he's being targeted by dark wizards because of his connection to Harry. Harry, being the heroic type, decides to fight to protect Teddy. He could use his magic skills and the support of his friends like Hermione and Ron to keep Teddy safe.
Well, it could be that Harry is facing some new dark forces in the fanfiction. Maybe he discovers a hidden group of dark wizards who are plotting against Hogwarts, and he decides to fight back to protect his school and friends.
The fanfiction could start with Harry being magically transported to Kaguya's world. He'd be confused at first but quickly realize he has to fight to survive. Harry's wand would be his main weapon. He might learn some new spells during the fight, perhaps a spell that can nullify Kaguya's ability to freeze time. As the fight progresses, both characters would start to respect each other's power, leading to an unexpected alliance at the end.