I would be stunned if Sanders attacked Clinton apart from not debating him before California. As I see it, Sanders attacks Trumps positions to show how he's more capable of landing the decisive blows on Trump and beating him in a general. Sanders is the only politician clean and progressive enough to test Trumps commitment to some of these US-worker-friendly economic policies he's advocated for but are now in the grey zone of things he walks back and forwards depending on who he's talking to. It will be interesting to see how Trump tries to shift his debating strategy from the primaries anyway.
Yeah I'm interested to see how Trump handles the debate. If he hits Bernie too hard and plays the man, then the danger is he'll reinforce his image as a bully and further alienate Bernie's supporters. On the other hand, everyone will be cheering on Bernie to land as many blows as possible, plus he has nothing to lose. Bernie's a skilled political operator and has deliberately held back from hitting Crooked Hillary too hard given, from my perspective, his goal is to push her to the left and then support her at the convention/general election. But if Trump fails to respond adequately to Bernie's attacks, it will damage his reputation as the tough guy who can handle ISIS, illegal immigration, terrorism and so on in the eyes of voters. Plus Trump hasn't really paid much attention to Bernie until now, and while Bernie has already made some harsh criticisms, Trump has let them all go through to the keeper so far (which, as we know, is certainly out of character).
That said, I think Trump will be able to easily navigate any testing by Sanders of his work-friendly economic policies. Trump is the master of a sound bite and, in any event, debates are poor venues for getting into detail about policy.