Are Mifune's claws specifically stated to be as potent as his sword? From your CS and your IC description of them, they sound like regular claws to me. Mifune is a speed-based character; haphazardly brute-forcing his way through defenses isn't the most viable of tactics imo, and it doesn't sound like his style either.

If Herriman's 'armor field' and his inherent physical toughness are to be respected and believed, I don't see the claws doing much of anything other than delivering a surprising blow that [i]may or may not[/i] briefly throw Herriman off balance/out of concentration. This could afford Mifune the chance to escape, re-obtain his weapon and reevaluate the present situation.