I have a feeling if Clinton wins the nomination, the GOP will have a better chance of winning the election. With Sanders gaining so much support, his losing would p*** off a lot of people, and those people may just decide, "aw **** it!" when it comes to the actual election.
For the debates, Clinton is probably going to try and say the same things as Sanders, counting on her popularity to carry her the rest of the way. If this happens, he needs to get aggressive and call her out on her B.S. He doesn't have to resort to mudslinging. But if Clinton says she supports 'A,' Sanders needs to jump in with, 'but you voted against it last term.'
He also needs to be a little more specific in his plans. Saying he wants free college and medical for all sounds nice, but he needs to say where it's coming from. Even explaining that, "If we pulled x% from our military budget, we could afford to cover 'y' and still have plenty of money to steamroll others if we have to, which WILL be a last resort.'
Oct 7 may be a big night for him; he can either leap past Clinton or fall back into her shadow.
I'm so hoping for the former.