Three responses from previous debates that will work against Trump: [View all]

[kentuck]

"There he goes again", when he tells an obvious lie. Which he will because he can't help himself.
It is an old quote from the Reagan/Carter debates.

"Where's the beef?", asked by Walter Mondale to Gary Hart. It totally disarmed Hart and may have helped Mondale get the nomination? Biden can use this line when Trump starts bragging about his wall.

"Mr Trump, you are no Abraham Lincoln", paraphrasing a LLoyd Bentsen quote directed at Dan Quayle, when Quayle tried to compare himself to JFK.

I am sure there are others that would be equally effective.