What level are you? I've done this several times at level 5-7 and one flame atronach usually takes out all three dremoras.
You have done that at level 5-7? O.o I died horribly several times when I first tried it, around level 20 or 25. The Dremoras die actually pretty easy, the trick is to dodge their fireballs. Summon something, Sanguine or Atronachs, get them distracted so the fireball doesn't hit YOU. Then charge them or take them out with a long range attack (bow in my case).
Then sneak down to take out the wizard guy.
Now, I leave that quest for last. I just did it with a level 50 Assassin. Snipe, snipe, snipe, snipe, all dead LOL