The fight is broken up into 2 phases, with 4 gotcha! mechanics you gotta watch out for in this fight. The TL;DR of how to win is a) never stop moving b) melt off cold stacks by standing in fire attacks and c) slot some kind of speed enchancing gear, either Alacrity weapons, Quickness signets, as well as bringing along a teleportation ability like Ley Line Stone gadget.
Phase 1
The masked woman is throwing normal attacks at you. She cycles through casting her Freezing manifestation, Fire manifestation, and Firestar. After you do enough damage to her, she casts Shatter Singularity and moves into Phase 2.
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Insta-death mechanic: Cold stacks. If you accidentally get hit by any of her Freezing manifestations here, you get stacks of cold on your character. If you reach 10 stacks, you die instantly, ending the fight. You can remove stacks though, by getting hit with fire attacks.
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Insta-death mechanic: Shatter Singularity. After you damage her enough, the masked woman throws out a huge AoE which covers most of the arena. You need to run for the edge and outrun the chalk marks on the ground, or you die. Depending on her location, the edge of the circle can go off the edge of the floating rock you’re on, which means death for you. Keep an eye on your surroundings, or try to keep her in the center of the rock to prevent this from happening. Also, even if you don’t get hit by her singularity attack, you still get some cold stacks at the end of the cast.
How to counter her attacks: For starters, keep moving constantly. She can’t catch you in any of the manifestations or firestar if you keep moving. Having Alacrity weapons or Quickness signet is a big help. Also, getting the Ley Line Stone gadget off the AH or from OD is a big help it for getting out of Shatter Singularity, the stone teleports you forward x meters. Having a Cruel Delight signet also helps you keep your health up, though you can use potions and healing abilities as well.
Phase 2
In phase 2 the Masked Woman stops using firestar, instead using a combo of highly damaging attacks. She also cycles through the manifestations. The green explosions can apply cold stacks as well as freezing manifestations. If you are getting high on cold stacks, jump into a fire manifestation for 1 second to “melt” them off. Keep up the DPS on the masked woman and she will run away at around 10% health, leaving behind a chest of rewards and an exit portal.
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Fold Spacetime. The masked woman warps to a point nearby you, and starts Fold Spacetime. This attack is actually 2 circles that only mostly overlap one another, there is a small gap between them. The “outer ring” of the attack is those exploding green circles. The “inner ring” is an AoE that spreads out from the masked woman. During Fold Spacetime the masked woman usually can’t be hurt, though you can trigger Cruel Delight healing, refill weapon energy, and apply debuffs by attacking her. Sometimes her shield doesn’t apply correctly and you can attack her. Finally, it looks like the inner and outer rings are cast on your position, but at slightly different times, so if you stand still both rings will end up overlapping. Keep moving!
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Destabilize Reality. Almost immediately after Fold Spacetime ends, the masked woman starts up some more green circle explosions. The first few seconds the explosions appear mostly right under you. The last few seconds, the explosions are in an ever-widening spiral out from the masked woman, kind of like the Machine Tyrant attack.
How to counter her attacks: Just like with Shatter Singularity you need to get out of the AoE of Fold Spacetime quickly. You can hide in the gap between the inner and outer rings of the attack, though it makes it harder to avoid the Destabilize Reality follow up when you’re stuck inside the outer rings. You can also start running away as soon as you see the masked woman warp, and just run out of the AoEs most of the time. Use Flicker, Ley Line Stone or a speed boost like Quickness to guarantee you’ll make it. As for Destabilize Reality, you don’t die instantly if you get clipped by one attack, but you do get hit with a hinder and cold stacks, and the explosions appear right under you, so one hit can end up being a death sentence. Again, just keep running as best you can, or save some teleport skills for those times when you get hit.
Videos
Here’s some videos at e17 with a variety of weapons:
E-levels
Increasing the elite level of the dungeon increases the masked woman’s health and damage done from regular and special attacks. It also increases the rewards from the chest.
But, around e9, Shatter Singularity starts applying cold stacks even if it doesn’t hit, to a max of 4 per cast on e17. Above e10, Freezing Manifestation will also apply cold stacks even if it doesn’t hit, to a max of 2 per cast on e17. At e12, the radius of the outer ring on Fold Spacetime jumps from 1 green circle width to 3, making it impossible to just run or walk through. Also while standing in fire melts off 4 cold stacks per tick under e10, at e17 it only melts off two cold stacks per tick.
Thanks to Jimmy, JohnLegrasse, Polutropon, Pilchenstein, Oneiva, Ceruleana, and Leogrim for information about the mechanics at higher elevels!
Hope that helps!