Hit him on policy over and over, that is how to beat him in 2020. Ignore the daily nonsense of stuff he says on Twitter. Fight for something, rather than "not as bad as Trump".
Trump sounded good to many who felt lost in the statue quo, but running a campaign and promising things is very different from governing. He is beatable, especially in various states in the midwest he barely won to overcome popular deficit with an electoral victory.
Democrats need to be shouting over and over a bold policy position such as "Medicare for All" up and down ballots. People still don't know what the left stands for and these tantrums about "The Internet Freaking Out" over some juvenile celebrity or petty social aspect is not the way forward.
2016 was a harsh lesson not to take people for granted or feel entitled to office, you have to earn votes.