Mothra is tough and while she doesn’t have the same physical strength that most of the Kaiju possess, she has a good amount of long range options. Mothra even has variations of herself like Mothra Leo, which amp up her attack power. Robin had a lot of gadgets, but they won’t really have an effect on this Kaiju. If Robin obtained many powers like Batman it could have been different, but that simply did not happen. Mothra wins.