Instant Karma

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

1. Mythic Difficulty.

2. Clear trash up to boss, and keep ONE Seacursed Soulkeeper alive, if you pulled him and can get out of combat and leave him till later that works too.

3. Begin encounter, don't reduce boss health too much.

4. Eventually boss will spawn Risen Warriors.

5. Grab your Seacursed Soulkeeper that you left alive, and bring him over to the Risen Warriors that spawned. Eventually he will kill them all with his Strike.

6. With the Risen Warriors dead by the Seacursed Soulkeeper, nuke the boss.

7. ?

8. Profit. Achievement Completed.

Okay everyone, this is how you get the achievement!
There is a mob in the trash pack of 4 before this boss called Seacursed Soulkeeper who uses a cleave called Defiant Strike. So what you have to do is kill the other 3 mobs in this trash pack and pull the boss with this guy alive. Just keep him alive until the Risen Warriors are spawned, then have the tank taunt him, get threat on all the risen warriors, and just let him use his Defiant Strike. He will kill them all, and then you just need to kill the boss to get the achievement. It's pretty simple :)

https://www.youtube.com/watch?v=eBJaxrdVhrM

You can do it more easily by running out of the dungeon once you cleared the 3 other thrash mobs. The Soulkeeper will reset and your group can walk around him to pull the boss. Then, your tank can just pull the Soulkeeper after the Skeleton adds are spawned.