What is hard and what means easy? The understanding differs at that point. What does a hard difficulty level for you mean? What is the main differ? Also there is the question how much difficulty everyone want. Otherwise how much simulation and is it funny?
I am oriented on a the line of precision. So harder means for me, that the requirements of my handling more and more nearing the line simulation. Sometimes this effects are not called as difficulty element. Or someone like effects that left the line.
I'm sure it is possible to make an AI in a short time so hard that nobody can beat him. But that would be only seek and destroy. Also sometimes there is no AI, what is the difficultyindicator then? When it is fair and when it left to be?
So, make the skill of an mp-gamer it difficulty?

I see a differ between a tuff and a well skilled AI.