So there is no explanation for Mirio's strenght making him able to knock hard rock characters like Kirishima. One wikia states he has enhanced strenght. Since he is able to de-solidify his body, and later solidify it to at least human level solid, can it be that he can solidify beyond human level and that makes him able to become harder and stronger to the point of knocking heroes with superhuman resistance? Similar to what another hero from another universe like Vision from Marvel who is also able to desolidify and solidify does? Is there any hint/statement in the manga or any guidebook about how much he can solidify his body after de-solidifying?