In a Batman vs Doomsday fanfiction, Batman could win if the story focuses on his preparation. He could study Doomsday's previous battles, find patterns, and develop specialized weapons or plans. For example, he might create a device that weakens Doomsday's molecular structure. On the other hand, Doomsday might win if the fanfiction emphasizes his raw power. He could simply smash through Batman's defenses and keep coming no matter what Batman throws at him.
I think Batman has a chance in this fanfiction battle. He could enlist the help of other heroes or use his detective skills to discover a hidden weakness of Doomsday. Maybe he finds a kryptonite - like substance that affects Doomsday. But Doomsday's ability to adapt and regenerate makes it a tough fight. The outcome would also be influenced by the mood and theme of the fanfiction. If it's a story about the underdog triumphing, Batman might win. But if it's about unstoppable force, Doomsday could be the victor.