It seems like the melee strength rings add to the npc damage as well as your melee units damage. I tried the first mission with the bear boss and a level 10 barbarian melee units and 2 of the melee strength rings that give 2 attack to melee units. With the rings on the npcs not counting the boss where doing damage of 6-12 to my units. And without the rings on the npcs not including the boss never did more than 1-4 damage.
Thanks for your comment, iI'm trying to fix the problem