When I first started playing it on easy, I couldn't imagine trying anything harder. Now after having completed all levels several times, I get bored on easy really fast.
The latest thing I've been trying is to make it through easy without killing anything. Well, make that without shooting anything. I only kill with a melee. Some areas won't allow you to advance until all enemies are dead.
I have not completed it yet but so far the most difficult thing to get past is yellow elite with the light sword/saber (whatever?). They are persistent and will chase you. Hard to get past them without killing them.