I'm not sure specifically as there could be many different plots in a 'Naruto Army of One' fanfiction. It might involve a character, perhaps Naruto himself, facing challenges alone against a large force, like an army, and having to rely on his unique skills and growth throughout the Naruto universe.
Most likely Naruto would be one of the main characters since it's in the Naruto universe and the title seems to imply a focus on a single character's story, which in this case is probably Naruto.
Well, he might use his powerful jutsu like the Rasengan. He could also summon his toads to help him out in the battle against the army. Maybe he uses his shadow clone technique to confuse the enemy and then strike them when they're vulnerable.
Some 'naruto one man army fanfiction' stories are really interesting. For example, there's a fanfic where Naruto is betrayed by his village but instead of being defeated, he trains in isolation. When he returns, he is like a one - man army. He has new and improved jutsus that are so powerful. He uses his speed, strength and intelligence to take on the entire village's forces that once turned against him. This kind of fanfic shows Naruto's growth and his ability to stand alone against great odds.
The idea of a character being a one - man army is exciting. In Naruto, we usually see him with his friends. But in this type of fanfiction, it's all about his individual strength. It gives a fresh perspective on his character.
Well, he might start by using his charisma. Naruto has always been good at making friends. He could reach out to old allies from his village, like Shikamaru and Choji. Also, he might convince some of the outcast ninja who have been mistreated in other villages to join him. His reputation as a hero who saved the world would surely attract a lot of people.
Usually, in these fanfictions, Naruto's character is depicted as having a more independent streak. Since he is a one - man army, he doesn't rely on his friends as much. However, his core values like his determination to protect those he loves still remain. His power growth is also often shown in a more rapid and extreme way. He might quickly master new jutsus that are far beyond his normal capabilities in the original story. This allows him to face large numbers of enemies alone and come out victorious. He is also sometimes portrayed as more of a lone wolf, but still with that Naruto - like charm and positive attitude.
Perhaps it's for exploration. Naruto is always curious about the world outside. The robots can be sent to explore dangerous or uncharted areas where it might be too risky for humans to go. They can collect information and report back to Naruto.
Maybe Naruto has some knowledge from an unexpected source. For example, he could have met a wandering inventor who taught him the basics of robotics. With his determination and quick learning ability, he then starts building the robot army. He might also use his chakra in some unique way to power or control the robots.