"Gosh, Bill Clinton is everywhere these days--I feel like I'm campaigning against him sometimes."
"This is a simple situation. If you want a Clinton restoration, you know who to vote for. If you want to move on, vote for me."
In general, Obama (or his surrogates?) could be phrasing everything so it's about both of them and not just her. "The Clintons want us to believe that..."; "The Clintons want to get back to the White House so that..."; And so on. That does three things at once: 1) It reminds people of the personal baggage they are carrying. 2) It reminds them that hers is a backward looking candidacy and 3) It reminds them that he is the only reason she's a candidate at all.
On another track, on the issue of change:
"The kind of change that we are talking about, historic change, isn't about knowing where all the little buttons and switches are so you can speed up or slow down or shift direction a bit. It's about getting a whole new vehicle!"
"If you've had too much of the wrong kind of experience you can't even imagine what real change would look like. Well (gesturing to the crowd), we can imagine it and we are going to make it happen."
With that change theme in mind, he should keep emphasizing the "new kind of politics" trope. There's no point in pretending that this is about his policy proposals. He should also keep hammering the "bipartisan new majority" trope--especially the idea that he will bring disaffected voters into the process. He has already done that--and he can pivot on that fact, use it as proof. And that, more than anything, promises victory in November.
"The Clintons want you to believe that I'm just a symbol of change, that I can't make real change. Well, just look at the turn out, look at all these first time voters committing themselves to our nation's future. That's real change, and believe me--it's just beginning!"