A Specter, Illuminated

This Is soloable! Here's how:

You don't NEED to be able to see the untargetable ghosts for this.

get the light (I had the one from Cordana's room and it didn't despawn on me on the way up like some people have said).
Next, you go pick a room that Cordana or Tirathon is in (doesn't matter which, I got Cordana first)
Cordana is in the small, seemingly empty room next to Glazer's room
Tirathon is in the small, seemingly empty room next to Ash'golm's room.
and you sit and wait.
Before too long, the attackable mob should spawn.
Kill it.
Wash/rinse/repeat for the next,
then simply head to the middle of the corridor for the Inquisitor.

I figured this out by chance while sitting in Cordana's room trying to google if this is not soloable when she spawned and started attacking me.

Was done on my 120, ilevel 322 assass rogue.

Easy Solo for Battle for Azeroth Level 120 and Beyond...

1. Mythic Difficulty.

2. I picked up Elune's Light at the statue by the Elevator that you take to get down to Cordana's encounter in the first place, I had already defeated Cordana, so Elune's light was easily accessible beside the elevator going up.

2b. You have to have Elune's light equipped at all times.

3. Go up the Elevator, proceed to 57.0, 42.6, the little sub-room off Glazer's room.

4. Run around in the room and eventually the mob spawns. Kill it.

5. Then head to 43.2, 63.3, the little sub-room off of Ash'golm's room.

6. Run around in the room and eventually the mob spawns. Kill it.

7. Then head into the hallway that connects both Ash'golm and Glazer's hallways.

8. This is where I found the Inquisitor. Kill him.

9. ?

10. Profit. Achievement Completed.

This is a weird achievement but it's pretty straightforward:

After the dungeon is done, grab the Light of Elune that you used on Cordana's fight and the other one just behind the elevator. With them, go to the hallway that is used to skip trash (connecting the eye guy and the golem guy) and there will be a ghost. Follow it, a mob will spawn, kill it.

Follow the hallway towards the north and Cordana's ghost will spawn, with ugly green floor with her. Use the light to keep it at bay while you fight.

After Cordana is gone for good, look around for a third enemy, who will "Steal your light" even though you don't have one. Kill him before he casts, or it will summon a BIG amount of adds.

The kill will grant you this achievement, which is required for Glory of the Legion Hero.