Trump did well for the first 30-45 minutes, and then she handed him his hat. He couldn't help but to respond to some of the things she accused him of, defending himself of the allegations. As soon as you go on the defensive in a debate, you're toast. You must stay on the issues and take it back to the opponent.
When she said Trump was originally for the Iraq War, instead of defending him and what he said, going on and on about what he said to Howard Stern, Sean Hannity, rolling ball bearings and talking about strawberries, he should have responded simply with, "I didn't vote for the Iraq War, you did." Boom. She's got nothing to say to that.