Yeah I made a post about this last week. The Briarhearts are insanely strong and swing so fast there is very little stopping them if you don't FUS-RO-DAH them or something. The only real way I found to fight them was to just sneak attack them and pray it one shotted them, because even in Dragonscale armor fully upgraded he could put me down in three hits.
That's what I have to do, fus ro dah and power attack them over and over.. generally they die before they can get back up. Something else I've noticed, I'm dual wield dagger/sword, and the "Critical Charge" ability is absolutely horrible. I'm still not even sure what it does other than make me clumsily swing around if I accidentally hit attack while I'm sprinting and drain my stamina. I'm not sure if it's a bug that you keep sprinting and swinging the more you click, but it's certainly annoying. I picked up the talent thinking it'd do one big hit, and then stop the whole thing, not force me to awkwardly sprint into my target in circles until I stopped sprinting all together.
@ ToJKa: The weird thing is I could wipe out camps of forsworn with no problem, not even sneak attacking. The second I hit 36, I wandered up to some forsworn and I was dead in seconds. Pretty confusing.. I don't understand what model Bethesda followed for enemies scaling this time, but it seems like they only applied it to the Forsworn.