once you get a 15 cloak tbh 5 mask is fairly easy at least for demo lock, there are only 3 fights with any real bit of difficulty, shaw alleria and rexxar.
and before anyone complains about umbric he is really easy as demo lock, you can implosion while he does his ice storm and the imps blowing up on him stops the ice storm so your initial tyrant is able to almost kill him on its own.
for shaw the only hard part is managing your pet in order to force shaw into a spot where you can attack him while avoiding eyes.
alleria is just spammed mechanics and as demo you have no interrupt so every polymorph cast goes through and stuns you.
and rexxar is just annoying especially with bad combos for madness.