The Magic Anomaly enemy in Skyrim is one of the few enemies that levels linearly with the player. I have finally encountered my first 10 during the College of Winterhold quest line.
I am a level 43 Stealth Archer with a dash of conjuration and the Magic Anomaly enemies are the most difficult enemies I've faced. Not the most deadly by far, but easily the hardest to kill.
At 43 I can only describe their health as close to a Frost/Blood Dragon, but much, much, much harder to hit. It takes a very, very long time to take one down and I'm looking for strategies centered around efficiency and time saving. Bows are out of the question, and my one/two handed are under 40.