They should do, through surrogates, what Rubio did. Poke fun at Trump personally. His hands, his business dealings,his cheeto face, his manner of speaking. Trump hates being criticized for his looks. Use the John Oliver moniker for him: Drumpf. And use it everywhere.
The objective is to get him outraged so that he lashes out. And then keep doing it. Rubio had him off so off message in one debate he had to reassure his supporters that "I don't have a problem down there."
The only way to handle a bully is to smash him in the face, twice as hard.