I got to do this last night and we wiped twice before we got it right. First time I'd wiped a DF story mission, leveled from 29 to 30 in the Manor.

That "Dark Mist" puts you in an awfully long stun and I could NOT seem to run out of the circle fast enough to avoid.

Third time I just stayed engaged with the Dark Lady and carefully husbanded my Shield Bash to the point I did very little actual attacking, just interrupting her big hitters (Void Fire, Void Lightning(?), Dark Mist) with SB while the two DPS (BRD and ACN) killed everything.