By Anistasia (5,722 – 1·9·41) on 2018/01/01 (Patch 7.3.2)
Elemental Escalation gave me a bit of trouble because I didn't have some of the pets suggested in other strategies.
I finally had an easy success with this combination:
Pandaren Water Spirit (Speed >281) (Dive and Geyser)
Tainted Waveling 1,1,2
Molten Corgi 1,1,1
Open with Dive, then cast Geyser.
Swap to Waveling. Cast Acidic Goo, Creeping Ooze, then Ooze Touch. Keep Acidic Goo up on him and fill with Ooze Touch until Waveling dies.
Swap to Corgi. Cast Superbark, Bark, then Puppies of the Flame will prevent One-of-Many from healing himself at the end of the fight. Cast Bark a couple more times and he should die.
I ended the fight with PWS and Corgi still alive.
** This worked 1st try for me when all others didn't (or I didn't have the stupid core hound)