I understand what you mean by wanting to keep things rare, however your methods of doing so are not actually making the challenge harder. All it is doing is making the challenge unavailable to be tried. Instead of making monsters harder to come in contact with and fight, why not make them harder so only the best can defeat them. Then players will truly "fear" them and have actually worked for their items. As of right now, it is your provoke macro, or worse, your bot, which really goes the work for you. From that point on, the fight is usually easy and some of the "best" HNMs can be beaten by small groups.