The difference is that while Trump has spent his campaign mirroring portions of the electorate, and speaking directly to voters, he has spent no time inflating himself as the most qualified Presidential candidate in world history, or being the world's expert in foreign policy, or domestic policy.
Hillary, on the other hand, has spent her entire candidacy pointing out that she is the most prepared person for the Presidency in world history.
Obama: Hillary is most qualified presidential candidate in history
She rolls out her experience as Secretary of State, and her foreign policy expert credentials at whim. She is keen to denigrate Trump as unknowing, or foolish compared to her incredible political savvy, and brilliance.
Hillary is uniformly reviled, and disliked. She is viewed as untruthful, and unlikable.
Trump is disliked as well, but this is qualitatively different. This comes in part from his bombastic personality, his aggressive statements, and the Hillary camps partially successful painting Trump as un-presidential.
All Trump needs to do is come out, present himself well, as a reasonable candidate whose ideas are reasonable, and have a reasonably good handle on the issues, domestic, and foreign.
Hillary must hit the ball out of the park just to meet expectations. Hillary has been saying she is the A+ student, so now she must prove it. How does one do this in a debate like this? She must have clever, novel, and obviously workable solutions to the majority of questions. This is like the comedian who sets himself up as the funniest man alive, he better have a brilliant riposte for every catcall.
The reason Trump will win this debate (in the eyes of the undecided voters, not the media - they are in the tank for Hillary, and will coronate her as the winner) is that he only needs to appear reasonable, and he will. Since he is likely to do better than just reasonable, it is likely he will win.
Hillary is equally unlikely to win because she has set her bar so high it will be impossible for her to achieve the necessary results. Instead she will likely look less than advertised. Hillary has never had a brilliant riposte, I cannot see her having one spontaneously on her own. And the canned ones hardly ever work for such terrible retail politicians. If she were Reagan, I might rethink that position, but she is not even Carter.
Hillary's real problem is she is a terrible retail politician, and she does not understand that while products can over advertise because they are only tried out after the sale, politicians cannot do this. They will be tried out during the debates. Over advertising only makes the bar that much more difficult to jump. Hillary set her bar as the most qualified candidate in world history, she must now jump that bar. Good luck.
Perhaps a single debate will not be enough to begin the shift, but if Trump ends up looking reasonable, and reasonably well informed after the three debates, it is over for Hillary.