For single post page, in the xtreme options > Xtreme Plus TAB, there is a box for “Per Post body Class ” you can use to put the class
I did not know what you were doing with the @ media rule, so disregard my remarks.
I did forget to mention that the rule to hide the sidebar on mobile for pages with the class would be
.is-mobile.myhidebar #primary-widget-area {display:none;}