The difference here between why Bernie may need a different approach compared to primary runs of the past is that there has never been a candidate with such a clear bias and disdain against them, coupled with an intentional silencing of their messaging and support that I can recall.
Trump may have been clowned, mocked, and ridiculed, but he got wall to wall media coverage.
Bernie is getting all of the ire that Trump did with none of the coverage benefit.
I think there's definitely room for him to approach the line more of questioning Warren's approach to certain issues and calling out the b.s. hurled in his direction without stepping over it.
If he truly does "welcome their hatred" like he quoted FDR, then embrace it to the fullest.