I think he needs to show that he can push back hard against a determined opponent, as he’ll have to do in the general election.
I think he also needs to show that he can deal with a ruthless opponent. Not all of Clinton’s attacks are against Obama’s qualifications. There’s the nagging issue of poaching delegates, changing the rules for FL and MI after the fact, and playing other technicalities. Fair or unfair, he needs to address these.