Put that way, the logic is fairly straightforward. You have to use the priest to break the barrier because it's the only purpose he can serve. Anyone can remove the barrier but only three can potentially attack the dark lord, because he must be attacked physically. Therefore it's logical to reserve the three that can attack physically to do so, and use the priest for the function that doesn't involve attacking, which he won't succeed at anyway.
no subject
I see what you've done. Now what's the catch?