The greatest weakness of his campaign is his lack of practical plans to make his promises happen. This is his vulnerability because if it can be illustrated that he has no way of implementing his promises, the support of those who are attracted by those promises will evaporate and he will be left with only his far left "true believers".
One example where I would hit him hard is on his free college tuition promise. I would ask him how does he plan to implement that. He would of course answer with this tax the rich scheme. I would then ask how is he going to get his plan through a Republican controlled Congress. To that the would give his standard reply that "the revolution" is going to put in place not only a new President, but also a new Congress. Then I would ask him how is that going to happen when he has contributed none of his huge stack of campaign contributions to down stream races and most of his Congressional colleagues won't support him because they're afraid that his presence at the top of the ticket will cost them their Congressional seats.
In closed primary states I would hit him hard on all of the important Democratic proposals which he refused to support and how many Democrats he ran against in his political career. I would bring up all of the times he criticized the Democrat Party and said that the two major parties were the essentially same, and that both were useless. I would emphasize that he isn't a true Democrat, that he is in interloper who is trying to steal the Democratic nomination.
Will Hillary go that far and risk alienating Bernie's supporters, probably not, but it would be fun to watch.