Well, in this fanfiction, Naruto might use his shadow clone jutsu to overwhelm the White Walkers. He could create a horde of clones and swarm them. Also, his Rasengan could be a powerful weapon against the undead. Just imagine Naruto spinning his Rasengan and blasting through a group of White Walkers.
In this fanfiction's plot, Naruto might team up with some of the local characters in the White Walker world. He could teach them some of his ninja skills in exchange for knowledge about the White Walkers. As the story progresses, Naruto discovers that there is a 'leader' among the White Walkers. He decides that to truly end the threat, he must take on this leader. He trains hard, combining his existing skills with new strategies he's learned from his new allies.
In this 'madara kills naruto fanfiction', the scenario might be set up in a way that the world around them has changed. Maybe there are new rules in the ninja world that Madara has adapted to better than Naruto. He could be using the political or social situation to his advantage. Additionally, the writer may have given Madara a different motivation that makes him more ruthless and determined to kill Naruto, and through a series of carefully crafted events, he manages to achieve this goal.
For such a plot to be possible in the fanfiction, it could involve some major changes in the Naruto world. The Akatsuki could have some new secret weapons or techniques. Also, Naruto might be in a situation where he is not able to access his full power, like being poisoned or something. And perhaps the Akatsuki are more united and desperate than ever to achieve their goals, leading them to take out Naruto who is a major obstacle for them.
Well, in a fanfiction, the writer can bend the rules of the original Naruto universe. Maybe Kabuto could kill Naruto if he somehow got hold of a super - powerful and unexpected weapon or jutsu that nullified Naruto's abilities.
Well, in a really dark and twisted fanfiction, Sasuke could be under some sort of mind - control spell. The spell could be so strong that it overrides his feelings for Naruto. He might then attack and kill Naruto without fully realizing what he's doing. But in the real Naruto lore, their friendship and mutual respect are so strong that this is highly unlikely.
One possible plotline could be that Naruto is somehow mind - controlled. Maybe an enemy uses a powerful genjutsu on him, making him see the Konoha 12 as threats to the village instead of comrades. This would lead to a tragic situation where Naruto, in his altered state of mind, attacks them.
One possible plotline could be that Naruto has to kill Momoshiki to protect the Hidden Leaf Village. Maybe Momoshiki has some new and extremely dangerous power that threatens the safety of the village and its people. Naruto, being the hero, steps up and in a fierce battle, manages to defeat and kill Momoshiki.
In fanfiction, anything is possible. Naruto has his powerful jutsu like Rasengan and Shadow Clone Jutsu. If he were in the Highschool DxD universe, he might use his speed and surprise factor. Maybe he could catch Riser off - guard and land a fatal blow with a Rasengan. However, Riser also has his own set of powers in the Highschool DxD world. So it would be a very interesting and intense battle if written well in a fanfiction.
One possible plot could be that Naruto is somehow corrupted. Maybe he's been influenced by a powerful dark force or a powerful enemy has tricked him into believing that everyone is a threat to the world he wants to protect. So, he decides to take extreme measures and kill everyone to 'create peace' in his warped sense.
It could be that there is some sort of misinformation spread by an evil organization. They create false evidence that Batman is planning to destroy Naruto's village. Naruto, being the protector of his village, decides to take pre - emptive action. He tracks Batman down and in a fierce battle, kills him. But then as he starts to investigate further, he discovers the truth and has to live with the guilt of killing an innocent (in this case, Batman who was misrepresented).