These guys are stupidly hard. I have Ebony superior armour on at 128, I'm at level 41 and the guy can kill me in one or two hits.

Don't think you that's rather crazy since a Dragon or Giant cannot even do that to me?

I've found some forms of magic relatively scary. I've run into a couple mages with shock spells that can practically kill me in two shots. Also, Elder Dragons are pretty tough for me, for some reason. Their flame breath will take about half my health away unless I interrupt it with a shout or shield bash. If they didn't take to the air repeatedly, allowing me to heal myself, they'd be even tougher.

Sneak, steal their Heart. They die.

well if your character is a thief or sneaky enough you can always pickpocket their Briarhearts and it will kill them instantly.

I'm almost certain they're a developer oversight, as they get ridiculous at higher levels. Seems like they must've scaled their damage too high. They can deal 300+ damage with one handed weapons, which they often dual wield.
Supposedly some in certain locations are worse than others.

More detail on them in a topic I made awhile back -
http://www.gamesas.com/topic/1326861-forsworn-briarhearts-bugged-or-very-odd-scaling/page__fromsearch__1

I seemed to have only just bumped into this guy when getting a Daedra artefact on Dead Crone Rock. I can avoid but damn.

Like Komodo said, if you can pickpocket their heart, they die instantly. Probably the best way to deal with them.

They do a [censored] load of damage to me on master, I use frost atronach against them though, drain their stamina. It's the triple spin dual wield attack that does the damage.

I can one shot them. Took a while to build my beast but at 120 with the big sword, x2 for a power attack and x2 for a critical they go down with an animation.

OK, just took him out using a dual attack, Ebony mace and Ebony war axe while using the elemental Fury shout.

My Ebony great sword with enchanted fire wasn't good enough and that's usually good enough to kill anything.