Yeah. I don't think a 2 person debate with Hillary is well suited for Trump. His shock value sound bites work and play a little better when there is a larger crowd of candidates sharing the time. But he will have to speak for a lot more time in a one on one debate.
Also making fun of a woman, even an unpopular one, will not play nearly as well as making fun of other men ('Little Marco' and all of his other little pokes) like he did in the primaries. He will have to stick to his 'Crooked Hillary' crap which will get old after he says it 15 times, which will take 15 minutes.
No matter how much you like Donald or hate Hillary, I don't think anyone is going to dispute the HUUUGGGEEE knowledge can depth of understanding gap between the 2.
He will not have a teleprompter. He will not even have notes. I don't even think the candidates know the EXACT questions. They may know topics, but not exact questions. There is literally NOTHING to help him stay on the rails. And he likely will not have the huge swarm of acolytes in the audience reinforcing his word vomit that he does in his speeches. Presidential debate audiences will applaud periodically, but are typically more serious and subdued.
If either of the 3rd party candidates get to 15% and end up on the stage, that would be a HUGE benefit for Trump.