All he (Mitt) need do is show up with his A-game.
The Obama campaign machine made a fatal mistake by completely vilifying Mitt for the last 6 months... Mitt showed up to the first debate, on the same stage with Obama, and (surprise, surprise!) seemed like a decent, rational, fair, and PRESIDENTIAL man.
Furthermore, Obama reminds me of GHW Bush in '92, carping about why the electorate SHOULD NOT vote for Clinton, rather than giving us reasons to vote FOR him (Bush).
The final nail in this coffin is the Benghazi issue. At the least, Obama looks inept; at the worst, deceptive (scapegoating the whole issue with a YouTube video)... For a man who campaigned on "new transparency" in Washington, he's going to have a hard time weaseling out of this one.