Souls of the Lost

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

1. Heroic Difficulty

2. Begin Fight, minimal damage on Ner'zhul--don't hurt him much.

3. Ritual of Bones begins.

4. Kill two Ritual of Bones Skeletons within 5 seconds of each other.

5. Kill Ner'zhul

6. ?

7. Profit. Achievement Completed.

This achievement is ridiculously easy with a Brewmaster Monk as the Monk can cast [spell=115080] instantly killing one of the required adds while it is at full HP while the rest of the DPS carry on as if they are not doing the achievement.

EDIT: Tried doing it today (Without a Monk), you MUST DO IT ON EVERY PHASE the adds spawn or else you will not get the achievement, we learned the hard way. The achievement description is extremely misleading.

EDIT 06/01/2015: Turns out it was a bug and has been fixed as of the January 6th 2015 hotfixes. You can now do it on one phase and still get the achievement

Ner'zhul: Credit for Souls of the Lost achievement should no longer incorrectly reset between waves of Ritual of Bones.

We attempted this by killing two adds at the same time once during the encounter and did not recieve the achievement upon killing the boss.

Either it's bugged or I am assuming you need to do it every wave.

UPDATE:

Apparently the achievement will reset if you let him cast Ritual of Bones again

Credit to JulianCamp

To clarify you just need to kill two adds within 5 seconds of eachother then defeat the boss before he casts Ritual of Bones again