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Heh, all these suggestions are okay, but I ended up killing her by teleporting her to the shore and having Bone Widow punch her for a couple turns; she won't kill it because it makes for a great charm victim. After that, just have every char out of range on the cliff, and have your archer rain Marksman's Fang on her. Walk a few AP away, reset fang and repeat.
Using Bless eliminates her pain aura; very helpful.
I ended up placed my Summoner (lvl16) on the second floor of the ruins and just far enough where I'm able to summon my Incarnate Champion. It triggers the battle and since the Summoner is NOT in battle, I'll buff and heal my Champion as it pummels the Witch. I did the same thing with the Harbringer of Doom. Soloed it.
Just make sure you pre-spread your party before engaging. She is very susceptible to holy/physical damage.
She gave me 43225 xp at level 16. Pretty nice
I wish you could begin a romantic relationship with her as an undead. She really gets me. :(
This fight is an epitome of dumb game design, an enemy who can one shot anything and ALWAYS goes first. To defeat it you pretty much have to resort to cheesing in some way.
Larian, please go replay Baldur's Gate II to see how to make difficult boss encounters without resorting to cheese, thank you.
God damn you're a salty *** nigga.
Beat her at level 14 in tactician mode. I'm perfectly fine with having challenging bosses in a game. What is not ok with is having a boss that instantly wipes your party forcing a reload and making your autosave useless since it reloads right before she obliterates your party unless you know what's she's gonna do beforehand.

The fight isn't even hard once you figure out a strategy, I spread out my party, used my warrior with Frost Armor on as a bait (she ignored him and went for my mage which survived due to his naturally high magical armor but the warrior would have survived too) and then destroyed her physical armor with my Ranger, which made her vulnerable to crown control, at that point the fight is just over, removing her aura with Bless is entirely optional if you deal strong enough physical damage because she should go down in two or three turns.
Living on the Edge+Forced Exchange works wonders on that fight