I take some issue with this concept:
Unless some of your players have _very_ specialized builds, AoOs only happen when the character in question takes certain actions (casting a spell, moving in certain ways, etc). Unless this NPC is an idiot, he'll realize that this is his primary weakness. Now, maybe the PCs can maneuver in a way that the NPC's only offensive options will all provoke AoOs, but it should be impossible to defeat a reasonably intelligent opponent who is only vulnerable when he takes certain actions. At some point he will realize that he will lose the fight and stop taking those actions (read: flee). And if the PCs can't hit him when he's not provoking AoOs, his flight will succeed.
Unless you want the fight to be resolved by a retreat or the NPC is very stupid (ie, animal intelligence if even that high), you are building a character that will require you use unbelievable tactics.