Well, on one hand, Hidan's immortality gives him an edge as he can keep coming back no matter how many times he's 'killed'. But Naruto is not just any opponent. Naruto has the power of the Nine - Tails and has trained with some of the greatest ninja. His speed, strength, and intelligence in battle are remarkable. He could potentially use his shadow clones to distract Hidan and then land a fatal blow. So, overall, Naruto has a greater chance of being the stronger in this fanfiction encounter.
Yes, in many fanfics Naruto is depicted as stronger than Kakashi. Fanfic authors often like to explore the growth potential of Naruto to an extreme level, surpassing his former teacher.
Man vs. Wild: Eat to Be Strong was a novel. It told the story of the protagonist, Xu Lei, who became stronger by activating the foodie reward system and eating. He could obtain different abilities by eating different foods. For example, eating snake meat could increase his strength, eating poisonous fungi could increase his poison resistance, eating ginseng could increase his life, and so on. With the help of the system, the protagonist became a man at the top of the food chain. Even dinosaurs would have to run for their lives when they saw him. The latest chapters of the novel included Chapter 65, Bamboo Discovered, and Chapter 64, Lei Shen's Physical Strength Explodes.
The elephant gun and the Barrett were two different types of weapons, each with their own characteristics and uses. The Elephant-hunting Gun had high lethality at close range and could kill the enemy's upper body in one shot, but its accuracy and stability might have some problems. The Barrett was an anti-material sniper rifle that was specially used to attack light armored targets and equipment with high accuracy and lethality. However, there was no clear information on which one was stronger. Therefore, it was impossible to determine which was stronger, the Elephant Gun or the Barrett.
The elephant gun and the Barrett were two different types of firearms, each with their own characteristics and uses. The Elephant-hunting Gun had great power at close range, but its accuracy was poor. It needed to be matched with a dead-eye lock to hit the target better. The Barrett was a large-caliber sniper rifle that was specially used for anti-equipment and long-distance shooting. It had high accuracy and lethality. Therefore, it was impossible to simply compare which one was stronger because their designs and uses were different.
Minato might also be a prominent character. As their teacher, he can influence the way Obito and Kakashi interact. In some fan fictions, he could be the one to help them resolve their differences or provide some crucial information about their past that changes their current situation.
One reason is Naruto's vast chakra reserve. His Uzumaki heritage gives him an almost unlimited supply, which can be emphasized more in fanfic. So he can keep going in a fight longer than Kakashi.
Some fanfictions might explore the emotional aspect of their fight. For example, how their past friendship affects the battle. Maybe Kakashi hesitates to strike a fatal blow because of their shared history.
In some fan fictions, there might be a plot where Obito has a change of heart before a major battle with Kakashi. He could start to question his motives and the path he has chosen, leading to a very intense and emotional confrontation with Kakashi that is more about ideology than pure combat.