8-B Saitama effortlessly one shots a High 8-C+ monster while sidetracked with rushing to the bathroom. He very likely takes care of Mugen in a few hits, while Mugen shouldn't be able to really harm Saitama, at least not any significant harm. Mugen's only advantages are range and skill, but that can't close the huge strength gap between them. I'm also not sure what Mugen's calcs are, but high end building level is up to 11 tons, high end City Block level is up to 100 tons, starting out at 11 tons for the absolute minimum. I'd go with Saitama low to mid difficulty.