I just killed her with my most recent build (e.g. a Blademaster), but man was it a harrowing experience. Now mind you this is a very sturdy build which can facetank pretty much everything else in the game (Fabius, Sentinel, Mad Queen), so her power pretty much took me by surprise.
I would guess she reflect and/or retaliates and that coupled with her Dot’s (bleed and electrocute), a high life steal% and good defenses just aren’t enough to offset the difference.
I wonder if Iron Maiden having retaliate damage correlates at all to the Diablo 2 Necromancer skill Iron Maiden being straight up reflect (retaliation) damage.