Who's Afraid of the Dark?

Don't be confused by the old comments. If you're a decently geared 60 in Shadowlands, just walk up to Cordana without picking up the light, and nuke her immediately. You don't have to worry about anything. You can nearly oneshot her.

I've compiled some information about this achievement - parts may be evident to some, but they are mentioned for completeness sake.

This achievement, part of the Glory of the Legion Hero meta, is obtained during the Cordana Felsong encounter (last boss in [zone=7787]). It must be completed on Mythic difficulty.

The ability to avoid for this achievement is [spell=197422]. When Cordana Felsong uses this, she spawns a line of spirits that march across the platform and both deal damage and knock back anyone they come into contact with.

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• This ability is not part of the encounter on Normal difficulty, in case you don't remember seeing it there.
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The successful completion of this achievement requires 2 elements:

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2. No one must be carrying the [spell=197941] during [spell=197422] phases.
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4. No one must be hit by [spell=197422] spirits AT ALL.
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The spirits from [spell=197422] are hidden, but can be revealed by using [spell=197941] to illuminate them. Obviously, this achievement requires that you not use that to help you.

If you have [class=12], as mentioned elsewhere, Spectral Sight can reveal the gaps. However, no particular class is needed to do this achievement.

When Cordana Felsong casts [spell=197422] each time, she fires a volley of arrows that mark where the spirits are spawning. There will be a gap in this volley, that will tell you where the safe space is to navigate each wave.

Therefore, by watching Cordana at all times, you can determine where the gap is going to be. If one person in the group is particularly adept at spotting these, he/she can put down raid markers to identify each gap.

Some further notes:

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• To get credit for this, the person carrying [spell=197941] must throw it onto the ground before Cordana Felsong casts [spell=197422], and must only pick it up after ALL of the spirits have completely passed through the platform.
  
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  • Picking it up too early will result in this achievement not being credited.
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  • Note that the [spell=197422] spirits will be visible when they pass by the [spell=197941] orb on the floor, which may be a little helpful.
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• Cordana will cast this ability for the first time at 40% health, and thereafter 70 seconds after casting it the first time.
  
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  • Ideally, using Bloodlust/Heroism/Time Warp after the first [spell=197422] can hopefully prevent another one from materializing.
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  • You can delay the [spell=197422] phase by avoiding bringing her to 40%, in order to first clean up any [spell=213583] that may be spreading across the platform.
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  • The person carrying [spell=197941] must pay attention to the boss' health, and be sure to throw down the light before she hits the 40% threshold.
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• Cordana spawns [spell=197422] in a particular way:
  
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  • The first time (at 40% health), she will spawn 4 waves of spirits; she will trigger each wave after the previous wave is halfway across the platform.
    
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    • During this phase, which lasts about 35 seconds, she is immune to damage.
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  • For the second [spell=197422] phase (if there is one), she spawns only 2 waves perpendicular to each other, but spawns both simultaneously rather than staggered.
    
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    • For the second [spell=197422] phase, Cordana is not immune and can be attacked (and hopefully killed soon after it has begun).
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This achievement can be cheesed in 8.2 if you're geared and have the Cyclotronic Blast red punch card. Just damage the boss down to just before she casts Creeping Doom (~40%), stop damage before she casts it and unload a full Cyclotronic Blast on her. If you get 1 or 2 crits during the blast it should burst kill her down before she even gets a chance to cast Creeping Doom. I did it as a 435 Outlaw Rogue. Classes with big on-demand burst like Glacial Spike could probably do this without the punchcard with enough gear