One possibility could be that Naruto is somehow transported to the world of Greek mythology where Olympus exists. Maybe his powerful jutsu are seen as a threat to the gods on Olympus. He could start by accidentally causing chaos in a nearby village which gets the attention of the minor gods. As they try to stop him, he realizes he has to fight back to protect himself and in the process, his attacks end up targeting Olympus.
His Sage Mode could give him an edge. In Sage Mode, Naruto has enhanced strength, speed, and perception. He could use these abilities to dodge the attacks of the gods and then counter - attack. For example, he could sense the weak points of the gods' defenses and target them with his Rasengan or other powerful jutsu.
I'm not entirely sure specifically as it's a fanfic. But generally, it might involve Percy having some extraordinary power or facing a situation that forces him to destroy Olympus. It could also be a really creative take on the existing Percy Jackson universe, where the author wants to explore a different and more radical storyline compared to the original.
One possible plotline could be Percy being betrayed by the gods and thus deciding to take on Olympus. Maybe he discovers some dark secrets about the gods' rule that make him believe destruction is the only way to bring true justice.
Obviously Percy would be one of the main characters. There might also be other characters from the Percy Jackson series like Annabeth. Maybe the gods of Olympus would also be important characters as they are the ones whose home, Olympus, is being destroyed.
Grover could also be a major character. As Percy's best friend, he would probably be involved in trying to stop Percy if he thinks Percy has gone too far. Or, if he sees the corruption in Olympus, he may side with Percy. Grover's connection to nature means he could play an important role in the events that unfold, like using nature spirits to either protect Olympus or aid Percy depending on his stance.
There could also be some ancient monsters or spirits that are tied to Olympus' existence. They might rise up to defend it because their own survival is linked to the continued existence of Olympus. These could be powerful beings that Percy has never faced before, adding an extra layer of danger to his quest to destroy Olympus.
It could be Sasuke. If he were to turn to the dark side in the fanfiction, his Rinnegan powers could be used to destroy Konoha. He has the potential to be extremely powerful and if he has some sort of vendetta or is influenced by some dark force, he might take his anger out on the village.
I'm not entirely sure as it could be a very specific fan - created story. It might combine elements from Naruto and the concept of the Blade of Olympus from the God of War series. Maybe it features Naruto characters in a situation where they encounter or use a weapon similar to the Blade of Olympus.