Trump will pull no punches, and Clinton is already not well-liked. She will seem weak; he will seem strong.
As I’ve said here, this election is going to be a big “fuck you” to politics, to politicians and to government.
By taking the high road, Clinton will lose ground every day a little bit, just like the GOP candidates who tried to ignore Trump lost ground. And if she tries to fight back, she will likely be painted as desperate and a shrew/harpy/bitch and thus will lose ground.
This election is Trump’s to lose, but there is nothing that will turn people away from him that aren’t already turned away. And there is much that will draw people to him, including the bandwagon effect: people like to back a winner, and Trump projects “winner” like it’s his hairpiece. Hell, he can’t stop talking about what a winner he is.