Using magic doesn't mean you can do ANYTHING. Can i materialize an atomic bomb up my opponent's rectum just because i can UES MAHGEKZZZZ? Of course not, unless i have such a spell. Can i teleport because i can use magic? Of course not, unless i have such a spell. Do you have it? I think not.

As for negating Mifune's speed, or turning it against him, i can personally think of a great number of uses for Herimans body manipulation. He can secrete slippery oils, create a smokescreen, trip Mifune with a tentacle, warp his arm to form a muscle cannon that shoots bones, alter his vocal cords to create a high frequency sonic blast, or simple wait until Mifune generates a lot of momentum, and then erect a spiked wall out of his chest, impaling him without even having to move.