Re Mitt Romney’s toughest debate, he probably can't "win" on details of foreign policy. However, the election is not about details of foreign policy. There are no votes in the Benghazi mess up and coverup, and nobody cares too much about the entire crack-up in the ME, or whether Obama hearts Putin.
I think Romney can make his mark on two themes. First, the importance that America remain the leader of the free world, and the most generous and helpful nation on the planet. Second, that America's beneficial influence is founded on two things: our ideas of freedom and the wealth born of (relatively) free markets. The Reagan approach, minus the Cold War. That leads back to talking about prosperity at home.
(If he has the nerve to talk about respect, and bowing to foreign leaders, that would be fun but would not win any votes. This is fun too: Chavez, Castro, Putin endorse Obama... )
That's his message, or should be.