My reasoning was centered around  Monika's ability to delete other characters.  If the deletion is instant, then Monika wins, but if  it takes enough time,  and if deletion has to be done all over again when interupted , then sonic could probably get in enough hits to win. This  might not be the physical power related answer you were looking for. But really, pitting  a  physically unenhanced teenager against a three foot tall super hedgehog that can go the speed of sound and faster seems unfair.

I'd like to hear your reasoning.