It is a nice idea to outclass him on policy, but Hillary is the wrong person to do that. The debate stage is not a lecture hall. You don´t have time to make long arguments. To convince people on issues you need the right image/credibility and deliver stuff on point. She can´t do either.
Talking about policy is playing defense, while handing over control to Trump. It won´t go horribly wrong for her, but I doubt that she´ll win over anyone. If she´d be 10+ points ahead, she should do that, but it is quite a gamble, when things a close.