He'd probably have trouble coordinating the two halves and dealing with their different thoughts and desires.
One challenge could be dealing with the child's past traumas. If the child has been through a lot, like being orphaned or facing monster attacks on their own, Percy would have to help them heal. He's not a trained psychologist, so it would be difficult for him to know exactly how to handle it.
This plot twist could cause a lot of chaos. One Percy might be more cautious while the other is more reckless, creating tension and drama.
It could lead to all sorts of wild adventures. Maybe the two Percys have to work together to overcome some huge challenge that one alone couldn't handle.
He may face challenges in the magical realm. The magic in the Percy Jackson universe might not interact well with his cyborg parts. For example, if he tries to use his powers while his cybernetic parts are active, there could be some unforeseen consequences. Also, he may be an outcast in both the human world (because he's a demigod) and the demigod world (because he's now a cyborg), which could lead to a lot of emotional and practical difficulties.
Well, he might face the challenge of leaving his mortal life behind. His relationships with his friends and family could be strained. He'd have to deal with new powers that are probably overwhelming at first. And he'd have to adapt to the politics and rules of the godly world which are likely very different from what he's used to.
In this kind of fanfiction, Percy might challenge Zeus over issues like the unfair treatment of demigods. For example, Zeus could be overly harsh on demigods who make minor mistakes while the gods themselves get away with much worse. Percy, being a hero, would stand up for his fellow demigods.
One challenge is blending the two different mythologies. Percy Jackson is based on Greek mythology while RWBY has its own unique lore. Writers have to find a way to make these two coexist without it feeling forced. For example, how do you explain demigod powers in a world that already has its own set of powers like semblances?
They could be used as a form of mind control. Maybe there's a villain who has the power to make the tentacles attach to people and influence their thoughts. Percy has to resist this control while also trying to free his friends who have been affected. This creates a unique challenge as he not only has to fight the physical threat of the tentacles but also the mental manipulation they bring.
One challenge Percy might face could be getting enough resources. In the north, materials for ship - building might be scarce. He may have to search far and wide for suitable wood or metal. Also, recruiting sailors in a cold and harsh northern environment could be difficult as not many people would be eager to leave the relative safety of their homes for a risky naval adventure.
One challenge could be dealing with jealous teammates. Since Percy is so talented in the water, some might envy his skills. Another is the pressure to always win. Everyone expects the team to do great with Percy as captain. Also, he might face difficulties in getting the team to work together if there are different personalities.