Batman has beaten gods with prep, but a 400-foot radioactive kaiju is a logistics problem, not a combat problem.
Batman's prep-time superpower is about exploiting specific weaknesses. Superman has kryptonite. Flash needs friction. Each Justice League member has a vulnerability that Batman's genius can target.
Godzilla doesn't have an elegant weakness. He's a 400-foot, 90,000-ton nuclear reactor covered in armor plating. He's tanked nuclear weapons. His weakness is... other kaiju? Oxygen destroyer? These aren't things in Batman's utility belt.
Could Batman, given unlimited time and resources, develop a weapon capable of hurting Godzilla? Probably. He has access to Wayne Enterprises' R&D and Justice League technology. But building a weapon that can hurt something that shrugs off nuclear bombs is a project, not a fight plan.
Batman has defeated beings far more powerful than Godzilla through intelligence and preparation. He has access to Justice League technology and Wayne Enterprises resources. Given enough time, he could develop a Godzilla-specific countermeasure.
Godzilla is 400 feet tall, weighs 90,000 tons, breathes atomic fire, and regenerates from nuclear energy. He has no exploitable weakness that fits in a utility belt. One step and Batman is a stain on the pavement.
Godzilla wins in any realistic scenario. Batman is the ultimate prepared fighter, but Godzilla is the ultimate force of nature. You don't beat Godzilla with contingency plans โ you evacuate the city and hope he goes back to the ocean.
Batman also fights
Godzilla also fights