Thinking about what Dogen told Sayid in Sundown (Season 6 #06) in that Sayid should stab MIB in the chest before giving him any chance to talk. Then Claire tells Jack in The Candidate (Season 6 #14) that he decided to be with MIB the moment he decided to talk to him.
What if its not just that Sayid had to stab MIB before he could speak but that it really meant that you shouldn't talk to him alone. I started thinking of everyone who has spoken to him alone... Claire, Sayid, Jin, Sawyer, Kate, Desmond, Sun and Jack. (Even Ben and Richard.)
What if the person who can kill MIB, needs to be a candidate who has never been pulled aside for a private conversation with MIB. Who does that leave.... Hurley! So if my theory is correct Hurley alone can kill him and he needs to do it with others around.