The Clinton v. Sanders dynamic is interesting because both candidates are almost universally liked by primary voters. Its really a contest of priorities. Clinton supporters are pragmatic, or even cynical. Sanders supporters are idealistic, or even naive.
Clinton isn't going to get voters to dislike Sanders. What she needs to do is change the mindset of the electorate. She needs people to vote with their head, not with their heart.
She needs to remind people how hard it was to get Obamacare passed, even with a Democratic supermajority. She needs to point out just how dysfunctional congress is, how it can barely pass a budget each year. Her message should be that it takes more than hope to enact change, and she doesn't want the next president to have to learn the lessons of the past 8 years all over again.
This is an angry and fed up electorate you are not to get people to vote with their head and not heart. Her issue is she is running as the status quo candidate in a change election. I think Democrats like Obamacare but they don't love it, a vast majority likely prefer single-payer and you saw how popular the public option was.