We think in that case we would basically only be acknowledging the possibilities between the "The attack either misses, is ineffective, or it triggers a counter attack from the boss." and "The attack hits with moderate effectiveness, as determined by the nature of their action." range. So, you can't get taken out, but you also can only deal moderate hits at best. You could still trigger a counter attack from the boss, but the damage taken from that is more flexible and free form than the 1-5% zone.

If you make an attack with this restriction just add a little OOC note to your initial attack tag.