Dwarven Spheres. I don't know how they do it, but they are insanely fast and strike fast and hard. At least for me...

They're harder to kill than Dwarven Centurions...
All you have to do is sprint past them and they stagger trying to turn around. Spheres don't even stagger when I use unrelenting force.

Dwarven Spheres are one of the few enemies that cannot fall down like most enemies. The Greybeards, Dragons, and Mammoths are the others. I think Dwarven Centurions are the same as well.. Never use Unrelenting Force on one of them though.

Dragons are easier to kill than mudcrabs. Need I say more?

If I could one shot dragons, I've say they are as easy as mudcrabs.

Dragons are easier to kill than archers, because at least the only stagger effect they can cause is by landing, which isn't often. Archers pluck you from miles away, with that stupid stagger proc'ing along the way.