My characters a 2hander playing on Masters. When fighting those 2H NPC's that can 2-shot you there's a lot of timing and back-peddling involved.
I usually open with that shout that staggers then land a power shot, which usually will stagger them again. After that you do your best to stay just out of their range as they flail at you; when they attempt a power shot of their own and miss they're open to another power shot from you, which will stagger them and give you some time to get back out of range.
Any normal swings you can't get out of the way of you can block and deflect and they won't do too much damage. If the NPC lands a clean power shot on you start running and healing, because if it didn't kill you outright you won't be in good shape.
The first bandit chief I ran into in that mission to recover a sword took a lot of trial and error, but now I know how to deal with that kind of enemy solo. It was pretty easy with Lydia tanking shots but I hate feeling like I had to rely on a companion for any battle, so I had her wait as I figured out how to win alone.