Hermione could be a great help. She is very intelligent and resourceful. She could help Harry find evidence against Snape. For instance, if Snape is suspected of cheating in grading, Hermione could use her knowledge of Hogwarts' grading system and records to dig up proof.
In this kind of fanfiction, perhaps Harry discovers some serious misdeeds by Snape that lead to his dismissal. For example, Snape might be caught unfairly favoring Slytherin students in a major way, like giving them unearned high marks in exams while unduly punishing Gryffindors. Harry, being the observant and just character he is, would report this to the proper authorities at Hogwarts, which could be Dumbledore or the school board. And this could start the process of Snape getting fired.
On the other hand, Harry could be proud of himself for standing up for what he believes in. He may think that he has made Hogwarts a better place by getting rid of a teacher who was not treating students fairly. But at the same time, he might have mixed emotions as Snape was also an important part of the Hogwarts environment for so long.
Well, it could be that Harry discovers some major misdeeds by Fudge. Maybe Fudge has been covering up dark magic activities within the Ministry. Harry then finds evidence and presents it to the proper authorities, leading to Fudge's dismissal.
Sometimes, new original characters are created in fanfiction to help Harry. These could be students from other wizarding schools who have unique skills. For example, a student from a school in a far - off land who has mastered a type of magic that is unknown in Hogwarts and uses it to help Harry in his fight against the dark forces.
Snape's help could have a huge impact on Harry's Hogwarts journey. For example, he might give Harry tips on dealing with certain dark magic threats secretly. Since Snape is a very knowledgeable wizard, his insights into the history of Hogwarts and the secrets of magic could help Harry better understand the school's mysteries and use them to his advantage.
In such a fanfic, perhaps Harry discovers some long - hidden, damning secret about Dumbledore. Maybe it's related to Dumbledore's past actions with Grindelwald. Harry, being the honest and justice - seeking person he is, feels compelled to expose this secret, which then leads to Dumbledore being fired from his position at Hogwarts.
Maybe Harry discovers something about his past, perhaps related to his parents, that only Snape can help with. Snape was closely associated with Harry's parents. He might know some secrets or have some information that could help Harry understand a family mystery or some hidden power within him.
Perhaps Harry witnesses Snape being extremely unfair to a group of students, like constantly giving them detention for no good reason. Harry decides to take a stand and gathers statements from the students. He then goes to the Hogwarts board of governors and presents his case, leading to Snape's dismissal.
The magical creatures could play a role in helping Harry. For example, the centaurs with their deep understanding of the stars and magic could offer their wisdom. The house - elves, like Dobby (if he's alive in the fanfic), might use their unique magic to assist Harry in a time of need.
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.