Duh, get creative, Dag. Mifune cannot dodge well without steady footing, as the more he accelerates, the more force is exerted by him, and thus, the more friction he need to retain balance. If you cover the arena in oil, this will significantly slow him down, while Herima himself can simply adapt by grwing spikes from his feet. A smokescreen created by vaporising some sort of liquid through a chemical reaction occuring within his body is feasible to offset Mifune's attacks, as he cannot clear it. If you use either the bone cannon or tentacle at point blank range, his chances to dodge are exceedingly low, as he doesn't know what exactly he will run into. You cannot really dodge if all you expected was a punch, but suddenly the arm bursts apart and showers you with shard of bone?
And regarding your use of magic, i'd still say it would be better if you specified what he can and cannot do. As of now, i have no doubt in Heriman's ability to destroy a planet with his magic, as there isn't even a specified upper limit. The atomic bomb also remains as a possibility, as does creating a good shalesomething movie.