One type of secret training Naruto did in fanfiction was related to his sensory skills. He could secretly train to expand his range of sensing chakra. By focusing and meditating, he tries to sense even the faintest chakra signatures from a greater distance. This way, he can be more aware of his surroundings and any approaching threats or allies, giving him an edge in battles and strategic planning.
In some naruto secret training fanfiction, the training could be centered around Naruto's connection to the Nine - Tails. He might learn how to tap into the Nine - Tails' chakra in a more controlled and efficient way without the negative side effects. This could involve some spiritual journey within his own mind - scape where he and the Nine - Tails reach a new level of understanding and cooperation, leading to a huge boost in his power.
It could be Kakashi. Since Kakashi has a wealth of knowledge and experience, he might take Naruto on a secret training journey. He could teach Naruto some advanced sharingan - related techniques that Naruto can adapt to his own fighting style, even though he doesn't have the sharingan himself.
They could do strength training together. Kratos could teach Naruto some of his combat techniques that rely on pure physical power. Naruto could in turn show Kratos how to use chakra for things like enhanced speed and agility. They might also practice fighting against each other's typical enemies in simulations.
He'd receive training in dwarven combat. This would include learning how to use axes and hammers effectively in battle. He'd also be trained in physical strength, as dwarves are known for their sturdiness. For example, he might have to lift heavy rocks or mine carts as part of his strength training. He could also learn about dwarven strategy, like how to defend a stronghold against invaders. And there would be training in dwarven crafts, such as forging weapons and armor, which could give him an edge in combat in a different way than his ninja training.
In some fanfictions, Deidara might give Naruto training on explosive jutsu control. Since Deidara is known for his explosive clay techniques, he could potentially teach Naruto how to better handle and utilize explosive elements in combat.
Kaguya might start with chakra control training. Since she has such a deep understanding of chakra, she could teach Naruto how to refine his chakra even more. For example, she could show him how to make his chakra flow in a more stable and efficient way. She may also use her knowledge of space - time ninjutsu to train Naruto in that area. Maybe she'd create special training grounds using her space - time abilities to test Naruto's reaction and adaptability.
He might start with basic earth manipulation. Just like how he learned to control his chakra in the original series, he would need to understand the flow of chakra in relation to earth. Maybe he'd practice making small pebbles levitate at first.
Well, he might offer ninja training. You know, teaching basic jutsu like the Clone Jutsu, or how to improve one's chakra control. It could also be about physical training, like the intense workouts Sasuke used to do.
He'd need to do a lot of strength training. Lifting weights, doing push - ups and sit - ups to build muscle. Also, he'd need to practice his balance, like standing on one leg for long periods or walking on narrow beams. And he should practice different punching and kicking combinations to make his attacks more fluid.
He could start with simple objects. Maybe he'd practice moving small pebbles around, focusing on controlling their speed and direction. Meditating could also be a part of his training, to clear his mind and enhance his concentration for better control over the mind - moving ability.