From playing this game for a couple days now, i think its weird that superior foes seem to take way less damage than if i were 1/2 lvls higher.
Even quite early in the game this is the case with the brown bears. They end up killing you with just a couple hits (which is totally fine, dealing more damage to "lower" enemies should work both ways, for player as for enemies)
But i dont think there should be a penalty for killing higher level enemies.
Imagine you get attacked from a bear in real life. Full armored dude with a knife would end up killing the bear (maybe) but if you get attacked and have nothing but a knife each stab might end up dealing the same damage (if you know where to stick it :D) and given that were dealing with a pretty old beeing thats been on this realm for who knows how long- chances are they know what they are doing! They are just in a weak body.
I get it from a design point of view, it makes the game much harder if you dont follow the correct path, but i usually end up going for "no challenge" enemies of my level. Maybe just have it as an option.