In some fanfictions, Harry might get Umbridge arrested by exposing her illegal and cruel actions towards students. For example, he could gather evidence of her misuse of power in punishing students with the blood - quill and present it to the proper authorities in the wizarding world.
It might be because of her anti - Muggle - born policies. Fanfiction often explores the idea that Harry discovers Umbridge's discriminatory plans against Muggle - borns. This is completely against the values of the wizarding world, and Harry, having Muggle - born friends, would take steps to expose her and get her arrested for such bigotry.
In some fanfictions, Harry might use his powerful magic skills, like advanced spells he's learned secretly, to outwit Umbridge. For example, he could create a series of complex illusions that confuse her and lead her to leave Hogwarts on her own.
She's often portrayed as extremely cruel. She uses her position of power at Hogwarts to make Harry's life miserable. For example, she might give him excessive detentions with the blood - quill, just like in the original books. And she'll try to isolate him from his friends and undermine his credibility among the students and teachers.
Perhaps in the fanfic, Harry teams up with the other students to stage a large - scale protest against Umbridge. Their united front and the chaos it causes might lead to the school board or the Ministry of Magic realizing that she is not fit to be in charge at Hogwarts. Or Harry could use his connections, like his friendship with Sirius or the Order of the Phoenix members. They could dig up some dirt on Umbridge from her past that makes it impossible for her to continue teaching at Hogwarts.
In certain fanfictions, Harry could get out of trouble with Umbridge through the power of truth - telling spells. He might be able to prove his innocence by using a spell that forces the truth to be revealed. Also, there could be instances where magical creatures come to his aid. For example, if he has befriended some of the magical creatures on the grounds, they could cause a commotion that diverts Umbridge's attention long enough for Harry to get away. Maybe a group of pixies start causing chaos in her office, and while she's trying to deal with them, Harry slips away unnoticed.
In some fanfictions, Harry might outsmart Umbridge by using his knowledge of magic in unexpected ways. For example, he could use spells that are not commonly known to get around her rules. Maybe he uses a charm to make her quill write false reports, thus confusing her when she tries to use those reports against him.
The development of their relationship in Harry and Umbridge lemon fanfiction can be quite diverse. It could be that Umbridge, in an attempt to control Harry more closely, gets drawn into his world in a way she didn't expect. Harry, on the other hand, might start to see through her tough exterior and find some vulnerability in her. This discovery could lead to a series of exchanges between them, some filled with tension, some with a strange sort of understanding, and over time, this shapes their relationship in the fanfiction.
It could be Harry Potter himself. He has been a victim of her tyranny and might finally find a way to expose her once and for all.
In some versions, Snape uses his knowledge of the Dark Arts and Hogwarts' secrets. He might set up a situation where Umbridge is lured into a restricted area of the castle and gets into trouble with some magical creature or ancient magic, causing her to be removed from her position.
One possible plot could be that evidence of her illegal use of power and abuse in Hogwarts is finally uncovered. Maybe a student bravely comes forward with proof of her cruel punishments and false accusations. This leads to the Ministry of Magic having no choice but to arrest her.