Alright, well that's progress. We've at least identified the issue. There needs to be further variance in the way specific enemies defend against attacks. Not just CC, but attacks in general. An easy but not very elegant solution would be to just grant immunities to certain keystone enemies. A slightly better, but not perfect, solution would be to have immunities based on race. A good example of this would be having creatures with more than two legs or none at be immune to knockdown. This sort of system makes it so that there can never be a true one size fits all answer but not in the best way possible, I'll try to think of a better way though.
