Aberrant Behavior

The old comments here makes it look trickier than it actually is. Nuke boss down to 25%, AoE down all the aberrations that spawn and then kill the boss. No bugs, it works. :-)

Updated comment as of Shadowlands 9.1.5

Just did this achievement no probs, don't have to take gear off or touch the boss. Simply aggro him, then twiddle your thumbs. It took him about 2 to 3 minutes, but he eventually started spawning the aberrations.

I just waited until there was just over 12 in the area, kited them away from the boss while he was doing a long cast, burnt them then burnt him.

Easy peesy :)

If you're going for Glory of the Cataclysm Raider, attempting to do this encounter in heroic and getting this achievement simultaneously, you might have a hard time. Here's a tip me and my friend discovered.

Since the two parts of this achievement are counted separate, killing the 12 aberrations and killing the boss, you can start the encounter in normal, nuke the boss to 25%, kill all the aberrations, run out, change the encounter to heroic, run back in and kill the boss, and you'll get both achievements.

Make sure to track the achievement so you can make sure you got credit for killing all 12 aberrations, and make sure the difficulty actually changes to heroic. You might get a message saying the party was recently in combat and you have to wait a bit.