One way Percy could defeat a boggart in a fanfiction is by using his friends. In his own world, Percy often relies on his friends for support. He could call on Annabeth, for instance. Annabeth is very intelligent and might be able to figure out a strategy. Maybe she could come up with a way to create a sort of magical shield around Percy, preventing the boggart from sensing his fear. Or they could work together to create a diversion. Percy could use his water powers to create a distraction while Annabeth figures out how to banish the boggart. Another option is that Percy could use his connection to Poseidon. He could ask for some sort of divine help, like a small earthquake to disrupt the boggart's form - changing ability, and then he could strike it with his sword.
He could use his sense of humor. Just like in the Harry Potter world, laughter can be a powerful weapon against a boggart. Percy has a witty side, so he could crack jokes to make the boggart change into something less scary and then attack it.
Percy has his speed and agility. In the fanfiction, he might be able to use his demigod reflexes to dodge all of the Avengers' attacks. He could then quickly move in for a counter - attack. Also, his sword, Riptide, is a powerful weapon. It can cut through almost anything. He could use it to cut through the Avengers' weapons or their defenses if he gets close enough.
Another possibility is that Percy discovers some long - forgotten weakness of Zeus. In the world of fanfiction, the author could create a scenario where Percy stumbles upon an ancient prophecy or a hidden secret about Zeus. This knowledge could give Percy the upper hand. He could then use his wits and his powers to exploit this weakness and defeat Zeus.
Percy Jackson is really good at improvising in battles. He might use the environment to his advantage. If they are in a place with water sources, like near a river or a lake, Percy can summon huge waves to knock Galbatorix off his feet. And considering Percy has faced powerful foes before, he won't be intimidated easily. He could also use his speed and agility, which he has developed from his years of fighting monsters, to dodge Galbatorix's attacks and get in close for a killing blow.
Perhaps it could be a vision of the world being completely overrun by the Titans or other powerful enemies and him being powerless to stop it. Percy has always been a hero, and the thought of failing in such a big way could be a terrifying boggart.
Percy could first create a whirlpool around the Wendigo, disorienting it. Then, when it's off - balance, he'd lunge forward with his sword. His ability to control water temperature could also be used to send blasts of scalding hot water at the Wendigo, weakening it.
If Percy Jackson were to face a boggart in a fanfiction, it could be really interesting. Percy is used to dealing with Greek monsters, but a boggart is a different kind of threat. It takes the form of your worst fear. Since Percy has a lot of fears, like losing his friends or failing to protect his loved ones, the boggart could transform into something really terrifying for him. He might initially approach it like any other enemy, but then realize that it's not as straightforward as slashing it with his sword. He'd have to figure out a different way to defeat it, perhaps using his wits and courage in a new way.
A character might use a powerful charm like the Patronus charm. Since the Patronus is a positive and powerful form of magic, it could potentially drive away the boggart. However, this might be a more difficult option as the Patronus charm requires a lot of concentration and a strong, positive emotion.
In a fanfic, Percy Jackson could defeat Thanos with his water powers. He might use the water around to create huge tidal waves and drown Thanos. Also, his speed and combat skills, which he has honed through many battles in his own world, could give him an edge. He could quickly dodge Thanos' attacks and strike back with his sword Riptide.
Percy could start to form new relationships within the Faunus community. These relationships could teach him different values and ways of life. Maybe he'd meet a Faunus mentor who would help him understand and control his new Faunus powers, if he has them. This would be a big part of his development, as he'd be learning from a completely different perspective than he's used to at Camp Half - Blood.
I think Thor would be really tough. Thor has his hammer, Mjolnir, which gives him the power of lightning. Lightning is a very powerful and destructive force. Percy would have to be very careful not to get hit by it. Also, Thor is a god in the Marvel universe, just like Percy is a demigod in his world. So they are kind of on a similar level in terms of their 'godly' status.