Being beaten by Magnezone (conditionally), Scizor, Lucario etc does not stop him from being an effective Mence counter. This is why you have team mates. Theres nothing that guarantees that a person using Mence will also be using Magnezone. Most people think Mence can take Registeel anyways, and Mence loses. Most teams try to guarantee more than one way to deal with one pokemon as well, if one pokemon is your only answer to something there is going to be problems no matter what you do because the opposition may have more than one way of dealing with said answer to that pokemon.