I don't know if this would even be possible, but how about making fear a root+ fleeing effect. The mob rurns around and tries to run but since he is firmly rooted he can't. Also make it so he can be attacked and the root/flee effect doesn't drop. In this case fear is not so much a cause the mob to panic and run away but totally terrify it into innaction. Possibly giveing the effect a shorter duration then MEZ to ballance it.