User blog comment:Godisme/Magical Barrier Particles/@comment-1359322-20131031161651/@comment-7418318-20131102085507

Maybe he's vulnerable right after reviving, or he needs a certain amount of time to "recharge" his lives and if he's killed several times in short succession he'll die for good.

If that's not the case, they'll probably have to destroy his book or some other power source to vanquish him.