I've never debated, but if I were the Democratic candidate in tonight's debate, I would counter Trump's lies very simply:
"that's wrong". Repeat over and over as necessary. Have your staff ready to post documentation on each time you say it. Then segue into "What I would...(say, do, propose,).
Don't engage in factual disputes, focus on positive plans.
[updated: so much for my advice on debating. :-( ]