In a Naruto - Humunculi fanfiction fight, Naruto could use his Sage Mode to sense the Humunculi's movements better. The Humunculi, on the other hand, would use their unique abilities like regeneration and alchemy - based attacks. However, Naruto's experience in fighting powerful opponents would give him an edge. He could also learn some basic alchemy during the fight if the fanfiction allows for crossovers in skills, which would make the fight even more interesting.
Well, one possibility could be that Naruto uses his shadow clone jutsu to confuse the Humunculi. Since they are used to a more straightforward combat style, the clones might throw them off. Naruto could then use his Rasengan to directly attack them. Maybe he would also team up with some of the characters from the FMA world who oppose the Humunculi.
It's really hard to say. Naruto has his powerful ninjutsu like the Rasenshuriken which can cause massive damage. But the Humunculi have regeneration abilities. If Naruto can figure out a way to completely destroy their Philosopher's Stone (the source of their power), he might win. However, if the Humunculi can use their alchemy to counter Naruto's attacks and get close to him, they could be a serious threat.
One possible plot could be that Naruto is somehow transported to a different timeline where he has to fight the Hokage for a misunderstanding. Maybe there's a villain who has manipulated events to make the Hokage think Naruto is a threat. So Naruto has to fight not only to prove his innocence but also to save the village from the real danger that the villain poses.
In a 'naruto fights alone fanfiction', it could be that Naruto is on a mission in a foreign land where he has no support. The local ninjas are all against him for some reason. He has to use his Nine - Tails chakra in a more discreet way as he can't afford to go all - out and draw too much attention. He might befriend some local animals or spirits who help him in a more indirect way, like giving him information about the enemies' weaknesses.
One possible plot could be that Zeref has a new target in the Naruto universe. Naruto discovers this and decides to confront him. At first, Naruto is outmatched as he doesn't understand Zeref's magic. But then he starts to analyze it during their battles and figures out ways to counter it. Meanwhile, Zeref is intrigued by Naruto's never - say - die attitude and chakra. The plot could also involve other characters from both universes getting involved, either to help or to cause more chaos.
Well, the plot could involve Naruto being in a post - apocalyptic world filled with zombies. His ninja skills would be put to the extreme test. He could team up with his friends like Sasuke and Sakura. Naruto would have to adapt his fighting style as zombies don't feel pain like normal foes. He might use his speed and agility to dodge the zombies and then use his powerful ninjutsu to take them out in groups.
It could also be a sparring session gone wrong. Naruto might be training with the Konoha 12, but his new and powerful techniques are too intense. This causes the others to feel threatened and they end up in a full - on fight. Naruto would then have to control his power and show his friends that he doesn't mean to harm them.
Well, perhaps Naruto finds himself in a world where the Laguz are being oppressed. He decides to fight for them because he sees their struggle as similar to that of his own village. He could use his ninja skills to protect them from any threats, be it other powerful groups or some sort of dark force that wants to take advantage of the Laguz.
One possible plot is that Naruto's brother is jealous of Naruto's status as a hero. He decides to challenge Naruto to prove himself. The fight might start with some basic exchanges of punches and kicks. But as it progresses, they both start using their more advanced ninja skills. Naruto may struggle because he doesn't want to hurt his brother too much, but he also knows he has to stop his brother's dangerous actions.
Well, perhaps the Uzumaki clan's secrets are being targeted. Naruto finds out that there are people after the Uzumaki's sealing techniques. He steps up to fight because he realizes the importance of his clan's legacy. He starts by training harder to master any Uzumaki - related powers he hasn't fully grasped yet. Then he embarks on a journey to find the source of the threat and take them on, all while trying to uphold the honor of the Uzumaki clan.
One possible plot could be that Naruto is mislead into believing his family has been possessed by some dark force. So, he has to fight them to free them. For example, an evil ninja might use a genjutsu on his family members, making them act against Naruto's will.