That was the point of my original comment.
For a post, Genesis will assign a class of postid-# where # is the ID of the post. For a page it will be page-id-#.
So you might start with a responsive theme and then for your IDX page with the iframe, in the responsive styling area, add page or post specific styling to style just the content area (the area with the iframe).
That keeps the bulk of your site and the bulk of the elements on that page responsive, but provides different styling for your iframes or anything where the style sheet approach won't work.