In a fit of nostalgia, or madness, I've been taking my 73 mage alt through some of the Gates of Discord zones with the intention of doing as much of the Breakdown in Communication quest as possible. Although my ranger has completed the quest I've never been DEEP into the Crematory since the required drop has always come from a named not too far in, guess I was lucky. A few days ago I had the ranger and mage in the Crematory and this Slime Devourer popped up on track. Playing with a new computer and I didn't have maps yet so after several false trails I found him up on the 2nd level (which I'd never heard of before) past where the actual crematory ovens are (ghastly looking place, 4 or 5 grim ovens and an atmosphere of hopelessness) all by itself in a small room up a ramp. The ranger vaporised it and it dropped nothing. Since the fight was over so fast and at the time it never occured to me to check the emotes, I'm not sure if it spawned 2 adds or if the 2 adds I got came from elsewhere, it sure looked like it was alone in that small room. First time I've had a GoD named drop nothing, very strange.
I looked all over for information on several named so after much time spent killing mobs in the crematory, the slime devourer spawned for me. It's location is first room in the center under water. Not sure is the ph is a slime but it didn't seem to move. hope this helps
One of the possible spawn points is where you described, exact loc for it is -700, -150. PHs been Grimy Tureptas and Sludge Lurkers. If you fight at spawn point, nothing will add.
PS: Need to kill 20 mobs to make PHs spawn first still...