It depends on the rules of the battle in the fanfic. If it's a battle of pure strength, Ash's powerful Pokemon like Pikachu in its evolved forms or his other strong partners could overpower Alexa's Pokemon. But if it's a battle that involves knowledge about Pokemon habitats and behaviors, Alexa could potentially outwit Ash. However, considering Ash's growth over the years, he has also gained a great deal of knowledge. So, overall, I still lean towards Ash winning.
In a Pokemon fanfic, it's really hard to say who would win between Alexa and Ash. Ash has a lot of experience from his long journey across different regions. He has a deep bond with his Pokemon. But Alexa is also very knowledgeable. If it was a battle based on strategy, Alexa might have an edge with her research skills. However, Ash's in - battle instincts are very strong. So it could go either way depending on how the author writes the scenario.