The ancient Sith could defeat Sidious by having a more intimate knowledge of the Dark Side's secrets. They may have known ways to manipulate the Dark Side in ways that Sidious, with his more modern approach, wouldn't expect. They could also have had stronger physical manifestations of the Dark Side, like enhanced strength or durability, which could help in a direct confrontation.
By using forgotten Dark Side rituals. Ancient Sith might have known rituals that could directly counter Sidious' powers. For example, a ritual to disrupt his connection to the Force momentarily.
Well, it's possible. If we consider the fact that the ancient Sith were the originators of many Dark Side techniques. They might have had a purer, more raw form of the Dark Side power. Sidious, while extremely powerful, was a product of a more refined and political Sith system. The ancient Sith's unpredictable and wild use of the Dark Side could give them an edge in a fictional battle.
Well, one way could be that the ancient Sith have a different code or view from Sidious. Maybe they see Sidious as a threat to the balance of the Force in a way that aligns with the Jedi's view for a short time. They might also have a vendetta against Sidious for some power - grabbing within the Sith ranks that the Jedi could use to their advantage.
Some Jedi would be extremely suspicious. They've been taught all their lives that the Sith are the enemy. They might think it's a trap by the Sith. For example, young padawans who haven't had much exposure to the complex politics of the galaxy would be very wary.
Well, first, Harry would need to study the ways of the Sith. He has to understand his enemy. Then, he could combine his wizarding skills with some new knowledge. For example, he might learn how to use his magic to counter the Sith's mind - control abilities. He could also use his Patronus, which represents hope and light, to drive away the darkness of the Sith. And of course, he'd need to find the Sith's weakness, like in the way he found Voldemort's Horcruxes.
One way Harry could defeat the Chitauri is by using his knowledge of magical creatures. He might find a way to use a spell that affects the Chitauri in a similar way to how he would deal with a dark magical creature. For instance, if there's a spell that can paralyze a creature, he could modify it to work on the Chitauri. Also, his ability to communicate with magical beings might come in handy. Maybe he could find a way to communicate with some sort of alien magic (if it exists in the fanfic) and use it against the Chitauri. Another aspect is his friendship with his Hogwarts comrades. He could call on them for help, and together they could use combined spells to overpower the Chitauri forces.
It could start with a strange rift in the space - time continuum that pulls Darth Sidious into the Naruto universe. Sidious would immediately start sizing up the situation, seeing the potential in the chakra - powered ninjas for his own power - hungry schemes.
Well, one way could be that Naruto, with his newfound Sith powers, starts to view the Hidden Leaf Village in a different light. He might see the shinobi system as a form of oppression, much like how the Sith view the Jedi Order in Star Wars. He could then start to secretly gather followers among those who feel disenfranchised in the village.
The plot could involve a Sith lord from the Star Wars universe arriving in the Naruto world. This Sith lord sees potential in Naruto and begins to train him in the ways of the Sith. Naruto, initially reluctant, starts to embrace the power as he realizes it can help him protect his village in a different way. But he soon struggles with the moral implications of using dark side powers.
Naruto could use his shadow clones to overrun the Justice League. He could make thousands of clones and swarm them. Also, his rasengan is a powerful attack. If he could get close enough to Superman or other members, it could do some damage.
In this 'harry flees the sith fanfic', the plot might involve Harry being in a world where the Sith are a new and dangerous threat. He flees because he realizes he's outmatched at first. Along the way, he might meet some allies who help him understand the Sith better and find a way to fight back. He could be on a journey to find a powerful artifact or knowledge that can stop the Sith once and for all.