I agree with basically everything in this NYT op-ed piece by Charles M. Blow from a week or so ago:
The con Donald Trump committed on his voters is slowly coming undone. He is not honest. He is not a brilliant deal maker. He is not even competent.
His entire life, Trump has sold shimmer and called it silver. It was and is all an illusion, a brand built on selling banality with braggadocio. He shaped vapors into dreams and delivered them to those hungry for a taste of the showy, hollow form of the high life he came to represent. He was successful at exploiting those with an ostentatious appetite for the air of success. Trump’s life story is a pyramid scheme of ambitions.
He took that history to a people struggling through a drought of opportunity and he exploited their weaknesses: a shrinking sense of economic security and growing nativist tendencies.
As much as I agree with Blow's argument, however, "You are a sucker who got played" is possibly the worst political message I can imagine. If Democrats or anyone on the left makes this argument, they come across as exactly the prententious jerks that Trump voters perceive them to be.
So what to do? I'm not sure.
While I do not have any specific, concrete suggestions as to how to do this, I think it comes down to providing Trump with every possible opportunity to expose himself as the failure that he is, and let voters figure it out for themselves, on their own terms.