Odin and Princess Birdie were two different types of novels, so it was impossible to make a direct comparison. Odin was a novel about Norse mythology, telling the story of Odin and other gods. The Little Bird Princess was a children's fantasy novel. It told the story of a timid girl who became a little bird princess in a magical country. The two novels were very different in terms of subject matter and audience. The readers could choose to read them according to their interests and preferences.
In a kratos vs naruto fanfic battle, Naruto might have a chance if he uses his strategic thinking. He could use his shadow clones to surround Kratos and then attack from all sides. But Kratos is no pushover. His experience in combat is vast. He has fought against opponents with all kinds of powers. If he manages to break through Naruto's clones and get to the real Naruto, it could be over. However, Naruto also has his sage mode which gives him enhanced strength and perception. So it's a very tough call.
One idea could be a battle in a neutral dimension. Kratos, with his brute strength and god - slaying weapons, faces Naruto's ninjutsu and his ability to summon powerful creatures. Another idea is that they are forced to team up against a common enemy that threatens both their worlds. They start off clashing due to their different fighting styles and beliefs but gradually learn to work together.
Well, it really depends on how the fanfiction is written. If the writer decides to give Percy some new powers or boost his existing ones, he could stand a chance against Kratos. However, Kratos is a force to be reckoned with. He's faced the most terrifying foes in his own universe. Percy is used to fighting monsters in the world of Greek mythology, but Kratos has fought gods on their own turf. So generally, Kratos has more of an advantage.
One interesting plot twist could be that Percy discovers a way to nullify Kratos' god - given powers. Maybe he finds an ancient artifact during the fight that has the ability to do so. Another could be that Kratos, instead of seeing Percy as an enemy, realizes they have a common goal against a greater evil in the fanfiction world and they end up teaming up.
Their interaction would be tense at first. Young Kratos, full of energy and unbridled aggression, might see old Kratos as a threat or an obstacle. Old Kratos, on the other hand, would be more reserved and cautious. But as they start to communicate, they would find common ground in their shared past and their connection to their family.
Another possibility is Heimdall. As the all - seeing guardian of the Bifrost, he would be aware of Odin's situation from the start. He could be the one trying to piece together what happened to Odin and prevent any disasters that might occur as a result of Odin's wipe. His unique abilities and loyalty to Asgard would make him a strong candidate for the main character.