Trump has never debated policy- just hurled insults, ridiculed, and called names. With his disorganized mind and incoherent speaking style- he doesn't stand a chance.
He's got more of a chance against Hillary than against Bernie. There is a style of debator who simply throws a ton of garbage against the wall, can overwhelm their opponent, and never really let them get on track. I've seen this work in some instances, and even very good debators can be thrown off by that style sometimes.
Trump's got a chance of using that against because of the almost infinite avenues of attack that exist. She's already disliked, so Trump won't lose much going after her hard. But Bernie is different. As nutty as he is, he's not a disliked person, and doesn't have the same horrible track record as does Hillary. So, simply smearing him won't work. What works against him is going after him on a practical policy level, but that's where Trump is weakest.
This is a really bad matchup for Trump.