Three Faced

The Black Temple was merely a setback!

Devourer of Souls will cast this once every 6 seconds or so. Thus, you'll need two dedicated interrupters. The spell cast is around 1.5 to 2 seconds, not 100% sure. Simply set up an interrupt rotation between two melee and you'll easily get this achievement.

Just soloed this as a level 85 Beast Mastery hunter with an avg. iLvl of 350.
I had a beetle as a pet, I let my pet tank the boss and stood at the max shooting range myself.
The boss didn't cast Phantom Blast a single time.
Mend pet was enough to keep the pet up.
Just ignore the ghosts hitting you and dodge the laser, maybe take a couple of bandages to be sure.
Might work with other pet classes too, such as warlocks.
Very, very easy achievement, atleast for me.

(First WoWhead post \o/)

Hope this helped someone.