In this kind of fanfic, Harry often uses his Gryffindor courage. He might have first located Hermione by following the distress signal that Hermione unknowingly sent out. Once he reached her, he could have fought off any magical creatures guarding her. For example, if there were mermaids trying to stop him, he could have used a charm to stun them briefly and then freed Hermione from the chains or ropes that were holding her. After that, he'd carry her back to the surface as fast as he could.
Well, in many fanfics, Harry might use his quick thinking and swimming skills. He could have found Hermione using his magical connection to her or by following the signs in the underwater maze. Maybe he used a spell to break whatever was binding Hermione and then swam her back to the surface.
Well, in many such fanfictions, Harry might use his magic skills. He could cast a powerful stupefy spell on the troll to stop it in its tracks and then quickly lead Hermione to safety. Maybe he also uses his quick thinking and knowledge of the Hogwarts corridors to find a hiding place where the troll can't reach them.
Well, he could use his magic skills. Maybe he casts a powerful spell on Ron to stop him and then takes Hermione to a safe place, like Hogwarts where there are protective enchantments.
In the fan fiction, Viktor might use his excellent swimming skills. He is from Durmstrang after all, which is by the Black Lake. He could have swum quickly to Harry when he was in trouble in the water, grabbed him, and swam back to safety.
Before needing to be saved, Hermione could be a damsel in distress waiting for Harry. Maybe she got caught by some magical trap set for the second task participants.
Maybe Harry uses his magic skills to immobilize Ron. He could cast a spell like 'Petrificus Totalus' on Ron right when he's about to attack Hermione, thus stopping the beating.
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
Well, perhaps he uses his influence in the wizarding world. Maybe he has some connections or knowledge that allows him to intervene and get Hermione out of the abusive situation. For example, he could use his status among the pure - blood families to put pressure on the abusers.
Well, in some versions, Ron might use his quick wit and knowledge of Hogwarts' secret passages to outmaneuver Draco. He could lead Hermione to safety through a passage that Draco doesn't know about.
In some versions, Hermione might use her advanced magic knowledge. She could have discovered a spell that counteracts the pull of the veil. Maybe she found a way to disrupt the dark magic associated with it and was able to pull Sirius back from the brink just in time.
In the fanfiction, Harry could outsmart Hermione by understanding people better. Hermione is often very logical and book - smart, but Harry might be more in tune with the emotions and motives of others. He could use this to his advantage in a situation where they are trying to solve a mystery or deal with a difficult person. He can read between the lines and make decisions based on his gut feeling about people, while Hermione is still analyzing the facts from her books.