The environment itself is a big obstacle. Tartarus is a place of extreme darkness and chaos. It can mess with Percy's mind, making it difficult for him to focus on his training. The terrain is also treacherous, with rivers of fire and bottomless pits.
He might face the challenge of overtraining or undertraining. Since he doesn't have a coach or a training partner to tell him when he's doing too much or too little, he could end up either exhausting himself or not making enough progress. In fanfiction, he could also struggle with loneliness. Training by himself means no one to share his frustrations or small victories with. This loneliness could potentially affect his mental state and in turn, his performance. Additionally, without feedback from others, he may develop bad habits in his training that could be hard to correct later on.
One of the main challenges would be the dangerous environment. Tartarus is filled with all kinds of monsters and hazards. Percy would constantly have to protect the child from these threats. Also, the lack of proper resources like food and clean water could be a huge issue. There's not much in Tartarus that is suitable for a growing child.
One major challenge could be the powerful and malevolent creatures that inhabit Tartarus. Monsters like the Titans or other ancient evils would try to stop him. They are much stronger than the usual foes he faces on the surface. Another challenge is the very nature of Tartarus itself. The place is designed to be inescapable, with its dark magic and chaotic environment constantly working against Percy.
He could also face challenges related to the knowledge required to use his power effectively. In fanfiction, there might be ancient texts or prophecies that he needs to decipher in order to fully understand and utilize his new powers. But these texts could be written in a long - lost language or be guarded by powerful entities, making it difficult for him to access the necessary information.
One challenge could be dealing with the trauma. Tartarus is a terrifying place, and Percy might have nightmares that disrupt his school life. He could also find it hard to relate to his classmates who have no idea what he's been through.
He might face mental challenges like PTSD. Tartarus is a horrible place, and the things he witnessed there could haunt him.
One of the main challenges could be the lack of immediate feedback. When training alone, Naruto doesn't have a partner to correct his mistakes right away. For example, if he's working on a new jutsu, he might not realize he's performing it slightly wrong until it fails completely. Also, he has to motivate himself without the competition or support of his peers. He might get bored or frustrated easily since there's no one around to cheer him on or push him further.
He could learn special combat skills. His dad, Poseidon, might teach him unique ways to use water powers in battle, like creating more powerful water whips or shields.
He could rely on his natural demigod abilities. His control over water, for instance, might help him find sources of water in Tartarus that are safe to drink or use for protection. Also, his combat skills, which he has honed over many battles, would be crucial in fending off the various monsters in Tartarus.
Well, one possibility is that Percy has to deal with the various monstrous creatures in Tartarus. He might form alliances with some of the less evil ones to maintain order. Maybe he uses his powers over water in unique ways in the dark and chaotic environment of Tartarus.