You say that, yet the fact that he was chosen to give the Republican response, coupled with him factoring into the GOP's plan to attract minority voters, means that he will be pushed as a potential candidate for 2016. Maybe he may run again in 2020 if he loses like Romney did in both 2008 and 2012.
To me, the focus shouldn't necessarily be to make him appear unelectable, but have someone preferable in the GOP look better by comparison. If Rubio is only pushed because of his ethnicity, then that's going to be a problem when it comes to being questioned in things where Rubio may not that much background knowledge or experience. My hope is that once his response is complete, folks can begin dissecting his response and see where there may be flaws. That or if there's anything you can punch a hole into. Same with Rand Paul, if only to know what he can improve upon.