It's hard to say for sure. If the fanfiction focuses on Hinata's growth, she could potentially defeat Pain through her own skills and some unexpected twists. But Naruto is also likely to be a major factor, so it could be a combined effort of both of them that overcomes Pain.
In this fanfiction, Naruto might be extremely enraged seeing Hinata in danger against Pain. Hinata, on the other hand, would likely show her newfound strength and determination. Maybe she'd manage to land a few surprise attacks on Pain before Naruto fully jumps into the fray.
The outcome in this fanfiction could be a draw. Naruto and Hinata would put up a great fight against Pain. Naruto would be all fired up, using his Rasengan and other ninjutsu. Hinata would be there, protecting Naruto's blind spots with her gentle fist techniques. But Pain is no pushover. His ability to control gravity and rain down destruction could keep Naruto and Hinata at bay, leading to a situation where neither side can fully defeat the other.
One possible plot twist could be that Hinata discovers a new power within herself during the fight against Pain. Maybe she can tap into some ancient Hyuga ability that even she didn't know existed. This new power could turn the tide of the battle and surprise both Naruto and Pain.
One common theme is redemption. Pain is often depicted as a character who can be redeemed through his interactions with Hinata. For example, Hinata's unwavering belief in the good of people might make Pain question his own actions.
One possible plot is that Pain targets Hinata to get to Naruto. Since Hinata has a special place in Naruto's heart, Pain might think that by hurting her, he can break Naruto. But Hinata shows great courage and Naruto comes to her rescue.
Hinata saving Naruto from Pain in fanfiction can take on many forms. It could be that Hinata has been secretly training and has a new set of abilities that she unleashes to save Naruto. This not only changes the course of the battle but also the dynamics between the characters. Fanfic writers might also add some emotional elements, such as Hinata's inner thoughts and feelings during the rescue. This makes the story more engaging and gives readers a fresh perspective on the Naruto - Hinata relationship.
In such a fanfic, it might explore the emotional turmoil. Pain's attacks on the village would not only be a physical threat but also an emotional one. Hinata could be in a position where she has to choose between her love for Naruto and her duty to the village. Naruto, on the other hand, would be dealing with his own feelings for Hinata while trying to defeat Pain. And Pain himself could be presented with more complex motives, perhaps having some sort of connection to Naruto's past that is revealed during the battle.
You can try searching on some popular fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of fanfictions including ones about hinata and pain.