What's Your Sign?

Simple explanation with photos:

1. Fight as usual for first Dark Eclipse.
Bring her to low health, wait for second Dark Eclipse.

2. When second Dark Eclipse goes off, grab buff by standing on the INNER ring's white moon runes on the ground.
Note which 8 of the 24 symbols around the room turn white momentarily. All 8 will be the same type - ear, bat, or reaper, and you'll note the white runes are all two runes apart.

3. The 8 white outer runes turn back to purple. Run over all 16 of the other two types of runes that didn't light up. Do not touch the runes that were white. Paladin bubble helps.
Once those 8 runes are the only ones left (they are purple now), kill the boss.

TLDR: Floor should look like this when she dies. 8 runes, all the same type that lit up white during the dark eclipse.
This last photo from a separate pull so the white runes were different from previous two photos.

Credit to Zillions, thought photos would help.

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

1. Heroic Difficulty.

2. Get rid of your pet, you don't need that thing running around causing trouble or distraction.

3. Begin fight, damage boss only very slightly, perhaps no more than 25-35%

4. Boss will cast Dark Eclipse.

5. Make sure your screen is zoomed out all the way, and take note of a single white rune that lights up along the perimeter, many will light up so choose one, and it doesn't matter which one.

6. You will notice that the purple runes are 2 for every 1 white rune. So, run over the 2 purple runes BETWEEN each white rune. Start at the purple rune beside the white rune you took note of.

7. Do not run over ANY white runes.

8. Once you complete the entire circle, you will end up perfectly beside the white rune you took note of at the beginning. Only white runes remain. If not, wait for boss to cast Dark Eclipse again and try again--Rinse and repeat the above.

9. Nuke Boss.

10. ?

11. Profit. Achievement Completed.

I tried so many different ways of doing this solo (waiting for second eclipse phase etc.) After probably 6 kills this is what finally worked.

1. DPS boss down to about 10%
2. When add pops, kill it before it gets to boss
3. When she casts eclipse, stand on a white moon in middle circle (any moon)
4. Look at which of the three types of symbol in the outer ring goes white
5. When the symbols all go purple, run over every symbol that DIDN'T go white (every third one)
6. Kill boss

ACHIEVEMENT!