Well, a common theme is the re - evaluation of Dumbledore's actions. Since he was such a powerful and influential figure in the original 'Harry Potter' series, in 'dumbledore gets owned' fanfictions, his actions are often put under the microscope. Also, the theme of betrayal can be common. It could be that characters he trusted turn against him, showing that his trust in others was perhaps misplaced. This can lead to his downfall in these fan - created stories.
It could be any fan who is interested in creating a story around Issei's downfall. There are many amateur writers in the fanfiction community who might come up with such a concept.
It could be about a character named Issei facing a situation where he is overpowered or defeated in some way. Maybe in a battle or a competition within the fictional world of the fanfiction.
I'm not entirely sure as I haven't read this specific fanfiction. But generally, in a 'gets owned' type of fanfiction in the context of 'Love Hina', it might be about Naru facing some sort of defeat or difficult situation. Maybe she loses a competition within the Love Hina world, or perhaps she has an embarrassing encounter that makes her look foolish.
I'm not sure which specific 'dumbledore gets owned fanfiction' is the best, but you can start your search on fanfiction platforms like Archive of Our Own. There, you can look for tags related to Dumbledore and stories where he is put in difficult situations. Just keep in mind that some fanfictions may deviate a lot from the original source material in terms of character portrayal.
Fanfiction websites like FanFiction.net or Archive of Our Own (AO3) are good places to start. They have a large collection of fanfictions for various series, including 'Love Hina'. Just use the search bar on those sites and type in 'love hina naru gets owned' to see if it exists.