Onslaught, who powers were stronger then Prof X fought the F4 andsome other heroes at the Baxter building. He easily took down Johnny Storm with telepathy, but stated that Lyja the Skrull alien brainwaves made it difficult for him to dominate her mind so he took her out with another power. Prof X is a talented telepath, but MM is one as well and J'onn has been alive a lot longer then chuck. It has been stated that Prof X can not manipulate any mind he comes across easily, and he does have difficulty with alien species he's never been in contact with before. I'm saying that while chuck may be more powerful then j'onn, the MM is close enough in power to hold Prof X off from controlling his mind while he lauches an attack with another one of his powers, like speed, strength, tk, martian vision, shapeshifing, or intangiability. Take your pick. I'm not saying that Prof X can't win in a battle of the minds with j'onn, cuase i think he can, but he needs some time to to adjust for J'onn's alien mind and time is one thing he won't have the luxary of dealing with a guy who has near superman strength and is faster then quicksilver.
Besides what is with all this telepathic battles and what not. Batman would simply have Plastic man deal with prof X. I forgot why, but he's immune to telapahty.