The Quintessence of Light is uber annoying because she heals too fricken much, and the only way to counter it is to hit her with so many attacks and so much damage that she can't help but die.
After some experimentation, I used a Ghostly Skull, Pandaren Water Spirit, and Chrominius. There are definitely a few other combos out there that can nuke her, but with my current pets and options, this was my best combo.
(see updated strat information at the bottom!)
Initially, I tried just the classic nuke combo of the Pandaren Water Spirit and Chrominius (Geyser + Whirlpool + Howl + Surge of Power), but after trying it a few times, she'd always have a little life left, which would allow her to use Cauterize and heal for a megaton. No, more than a megaton, Like a quintessentalton. Yes, I know that's not a word.
Therefore, I needed to find an additional boost to my damage or something to put a dent in her HP before giving her the one-two-Chrominius punch. First, I tried the Nether Faerie Dragon with Life Exchange. Worked decently, but she'd still heal too much in the interim before I could get the Pandaren Water Spirit and Chrominius out to play.
So instead, I decided to use the Ghostly Skull with Unholy Ascension, which increases the damage taken by the target by 25% for 9 rounds. The Unborn Val'kyr also has Unholy Ascension (and Curse of Doom to boot), but my Val'kyr is only a level 19 green (yes, I know, I'm behind the times on this one!), so I needed to use the Ghostly Skull... which worked out just fine.
My series of attacks included:
Ghostly Bite > Unholy Ascension > Geyser > Whirlpool (switch to Chrominius) > Howl > Surge of Power.
Part of the reason why this works is because Geyser stuns the Quintessence and you can get in that final Surge of Power to finish her off without her healing a megaton.
This series of attacks should finish her off neatly each time EXCEPT if your Pandaren Water Spirit gets hit six times (over two rounds) with Fire Quills. It just does too much damage and your Spirit will die before getting that essential Whirlpool off. For this to work, you MUST use the series of attacks I outlined above, or the Quintessence won't take enough damage.
If you notice in the screenshot below, my Water Spirit was hit just twice during the first round it had with the Quintessence. This MUST happen.
Unfortunately, if your Pandaren Water Spirit bites the dust before you can cast Geyser AND Whirlpool, it's impossible to kill her. Which sucks because I really don't like starting battles over because of RNG.
After beating the Quintessence initially on the first try with the Ghostly Skull, Water Spirit, and Chrominius, I had a ton of bad luck on further practice battles (I tested this combo at least a half-dozen times). My Water Spirit just kept dying from Quills, which has a "chance" to hit 2 or 3 times, based upon my team's speed. Unfortunately, my Spirit died over and over again because of this "chance" hit.
If I had to guess, I think using Curse of Doom and Unholy Ascension could fix this problem by ensuring that Curse of Doom would go off with Geyser, which would mean Whirlpool wasn't necessary, but I can't test that for sure since my Val'kyer isn't leveled up yet. I'll fix that soon, I swear!
December 17, 2014 Update!
Now that I've got my Val'kyr above level 20, I decided to fight the Quintessence again with the following team, and it was much less RNG-filled.
My recommended series of attacks would be:
Curse of Doom > Unholy Ascension > Geyser > Whirlpool > Howl > Surge of Power
Yes, I believe it's best to kill the Val'kyr on round two and bring out the Water Spirit immediately and then switch to Chrominius. I tried using the Val'kyr a few extra rounds until it died, and I just ran into problems. Also, 90% of the reason this sequence works is because of the stun from Geyser. This allows your Chrominius to get that final Surge of Power off at the end where the Quintessence isn't able to heal.
And, FYI, yes, my Unborn Val'kyr was only level 23 when I employed this strat.
