Trump certainly doesn't not have a command of policy issues, especially foreign policy. He's done ok in the debates, but a major factor there is the sheer number of people. There not much time for each person, and so many different battle lines that individual failings like that can be minimized. One on one, he's not going to be able to get away with saying things like "I don't understand foreign policy, but I'll be the best in the world by the time I'm elected!" Hillary will be the obvious "serious" choice, and I'm not sure where Trump gets an advantage on attacking her that hasn't already come out.
I'm also not convinced that the public, the ones that don't already support Trump, will be able to stand his personality for 2+ hours at a time. Hills isn't exactly a charmer, but she'll certainly look better by comparison to Trump.