1) by laughing. he's laughed off a lot of what hillary has said.
2) satire. the speech bit, for example.
3) by repeating the nonsense in an incredulous tone, then launching into talking points.
so, for example, let's say the topic of global warming comes up. the donald thinks it's a nefarious plot by the chinese to crash the us economy. what, exactly, could bernie say on a podium in a minute or so that is going to address this? he won't. rather, you're going to get...
"...you think it's a plot by the chinese. ok. but, i think an economy where the 1% control...."
so, what is a debate, in context? it's impossible, because trump is consistently incoherent. while the juxtaposition may help in defining just how ridiculous trump really is, and turn off conservatives that are warming to him, it's hard to see how it can present sanders as anything other than confused, perplexed and flummoxed. if this is your first exposure to sanders, there's a good chance he's going to come off as a stammering old man.
"you...you want...you want to give nuclear weapons to south korea !? i...i...in an economy where the 1% control...."
so, what's actually going to happen, then, is not a debate. what's going to happen is that sanders is going to get a prime time tv slot to stump over, while he shrugs incredulously at the incomprehensibility of trump's positions.
no major network will actually let this happen.
https://www.youtube.com/watch?v=2W2v23m8RA0
i pointed this out earlier in the campaign: trump's irrationality is actually an advantage, if his opponent is rational. the reason is that rationality requires predictability.
the best example of this is reagan. it's widely understood that reagan forced the russians to stand down [i don't mean the coup, i mean collapse, i'm talking purely about the military] by increasing military spending through the roof. he then gets credit for "winning the cold war". there is actually a kernel of truth in this, but it's less understood how the russians reacted.
what the russians realized was that reagan was behaving irrationally relative to all their theories and models. you had generals on both sides running these game theory simulations against each other. one of the most famous results of this was this idea of mutually assured destruction. but, it's broadly acknowledged that one of the major reasons that nuclear war didn't happen was that both sides assumed the other would always behave rationally. this both led to unchallenged escalations and withdrawal decisions.
so, when they became unable to understand reagan as a rational agent, they had to stand down - because they could no longer predict his behaviour by assuming he was rational.
nobody should suggest that the lesson is that it is a good idea to elect irrational people to confuse the country's adversaries. rather, we should be grateful that the russians did not become unpredictable and irrational, but rather remained rational even in de-escalation.
but, it's going to put bernie in the same quandary on question after question. he can't argue with absurdity - and especially not in a few seconds at a time. rather, he's going to have to stand down over and over again.