Ok. Here's the situation:
I have infantry units in advance of a mixed artillery / infantry group. My opponent advance an infantry group on me but can't reach my mixed group because it is intercepted by my infantry units in front of the mixed group. Nevertheless my mixed group is under attack and loses both artillery units and some infantry all while being interdicted by my advanced infantry units.
How is it possible for my opponent's infantry units to cause casualties to my mixed group? Is there some rule stating that if my artillery can cause casualties to his infantry that his infantry can cause casualties to my artillery? I thought the advantage of artillrty was that it could bombard without taking losses.