Mifune is hitting the equivalent of enchanted steel plate with an extra layer of protection at least an inch thick. His claws aren't enchanted, or modified, they're literally just claws. He also isn't the strongest. Sure speed might be a factor, but there comes a certain point where it no longer matters how fast he's going. In fact, in this situation, top speed is the worst thing ever. 

Also, if Mifune's hitting herriman to the side, and Herriman's holding his sword out, Mifune would have to go under Herriman's arm and tackle him like that if he doesn't want to get stabbed. This would give Herriman plenty of opportunity to reverse the tackle, as Mifune would probably be distracted by having all of his fingers and nails broken at the same time.