Black Rook Moan

Upon reaching 50% health, Amalgam of Souls will start casting "Call Souls". This will spawn eight Restless Souls on the edges of the room, that will start slowly moving towards the boss. If they reach him, he'll get a damage increase for his group-wide damage cast.

If a Restless Soul is prevented from reaching the boss for 60 seconds after "Call Souls", it will turn into a Frustrated Soul, which is the same thing but with a red tint. The goal of this achievement is to kill that Frustrated Soul.

The souls are vulnerable to most kinds of CC, including druid's Entangling Roots, which makes holding the soul a matter of re-casting the spell once every 30 seconds.

On our first try, the soul we were holding disappeared for no obvious reason as if it died, even though it was at 100% and nobody was near it to accidentally cleave it or something. It happened when we pushed the boss below 15% health. I don't know if that could have caused it, but when we held damage at 25%, we had no issue. Might just have been some kind of bug though.

Here's a video of me and a PUG doing the achievement (apparently posting videos is a thing people like to do here).

Priest's [spell=9484] does not work on these sadly.

Solo'd today as a feral druid (although any spec would work)

Once he did Call Souls I picked one of the Restless Souls and rooted it, just stand there and keep it rooted until it changes into a Frustrated Soul, then kill it and then the boss.

Easy!