Let's talk about how to properly prepare yourself for this encounter. First and foremost, realize that spell power is the top stat for this fight. You'll have infinite mana from the stacking Emerald Vigor buff, which means that intellect and MP5 are pretty much useless. We need Emerald Vigor to inflate our healing amounts to make this fight possible, and so SP scales the best with the healing boost. While haste is definitely good, you'll also only be healing one target, so there's really no reactivity needed. Now normally I would recommend SP trinkets for this fight, but realize that our popular Talisman of Resurgence trinket provides nearly 600 SP for 20 seconds, which means we can pop it during our massive healing phase. This makes it a worthwhile trinket, and I would then pair it with a Sliver of Pure Ice or Solace of the Fallen.
I'd also recommend gathering a few Seal of Light and Seal of Wisdom glyphs while you're in town. You can usually get these made pretty cheaply, and just swap in Seal of Light for Valithria, and swap back to Seal of Wisdom afterwards. The glyph will be very handy when you're first learning this fight, though it's not strictly necessary once your group is comfortable with the encounter. Make sure that you actually use Seal of Light on the fight though, it needs to be active for the glyph to work, and I've often forgotten and used SoW with the wrong glyph. Speaking of glyphs, make sure you're using Glyph of Holy Light, as it is mandatory for healing this fight the right way. You'll want to have a spell power flask ready to go, possibly grab a Potion of Wild Magic, and finally go buy a Libram of Veracity. They're only 25 Emblems of Triumph, which you likely have a ton of, and they'll boost your SP by a decent amount.
Let your inner DPS class take over
One of the biggest challenges of Valithria is to leave the healing mindset behind, and enter into the head of a DPS class. Our healing per second must become the primary thought in our head, with every decision based around the idea of maximizing our HPS. As holy paladins, we are uniquely suited to healing Valithria, and so we're usually tasked with that job exclusively. In the 10 man version of this encounter, you can successfully heal her from 50% to 100% by yourself, provided you set aside your flashing grid of raid unit frames and get into the HPS zone. You must leave the raid healing to your other healers on the outside, as you have been chosen for a higher calling. Similar to Professor Putricide and the abomination, you are separated from your group most of the time, and your success or failure becomes the paramount point of the encounter.
First, always remember that you have infinite mana on this fight. Recast Beacon as often as you see fit to guarantee 100% uptime, don't cast anything besides Holy Light (or maybe Lay on Hands), and never stop healing. You're going for all-out HPS, don't waste time with Sacred Shield or Flash of Light. The proper healing tactic for this fight is to put your Beacon on Valithria, stand near her body, and heal yourself. This will produce a large Glyph of Holy Light proc on her, as well as replicating the entire heal to her via Beacon. Glyph of Holy Light dips into the healing modifiers more than once, since it's based off your original huge heal, and again since the glyph heal itself is modified. I've had the glyph do as much as 30% of my overall healing on this fight.
Open the fight by popping all of your cooldowns and practicing your maximum HPS burn. Use Avenging Wrath, any trinkets you have, Divine Illumination (if you have your 2 piece), and spam Holy Light on yourself with Valithria under your Beacon. Don't worry about your mana at all, burn through it all if you need to. You can get a few percentage points of healing done here before the fight even starts, and this will buy you a ton of buffer room later on. You'll be focusing on healing Valithria after each portal phase, I purposefully try not to heal the raid at all. The healers on the outside expect to be doing it alone, and the quicker that the boss is fully healed, the less healing they have to do as well.
The dream portals
Hop into the green portals as soon as possible, as the Dream State timer starts the moment they're available. Once inside, immediately fly upward and begin collecting the Emerald Vigor clouds. The dream phase will take a few tries to master, so focus on getting what orbs you can first, and then worry about perfecting your flight path later. Be sure to work with any other players who may be in the dream as well.
I consider a 5-cloud portal phase a success, although you can get around 10 per phase once you really know the fight well. Anywhere from 5-10 will have you getting plenty of stacks to end the encounter before the soft enrage. The key to this encounter is refreshing your Emerald Vigor stack every time you enter and exit the portal. Once inside, immediately grab a cloud to refresh your stack. Try to hit a cloud during the last 2-3 seconds of Dream State to ensure it lasts as long as possible outside. They recently buffed the length of Emerald Vigor to make this easier, but it still takes some practice.
Between portal phases, I like to rebeacon Valithria, stand next to her, and spam Holy Light as usual. If done properly, you can heal several % per phase this way, instead of trying to heal her all at once during a Bloodlust phase. The one catch is that you'll want to be refreshing Judgements of the Pure for maximum HPS, so try to judge one of the adds at least once a minute to maintain the buff. Otherwise, just chain-cast Holy Light while looking for the next portal spawn area. You should be looking to have the boss at about 75% life once you have 30 or so stacks of Emerald Vigor, and that's when you should start preparing for the HPS burn phase.
All cooldowns activated
Call out the HPS burn phase so that the rest of the raid can clean up any Suppressors that were left alive. The moment you drop out of the Dream State phase, rebeacon Valithria, make sure JotP has plenty of time left, and pop all your cooldowns again. Spare no expenses, pop absolutely anything you have to use. Have your shaman pop Bloodlust, possibly get a Power Infusion from a Disc Priest (if you don't have Bloodlust), or even score Focus Magic from your mage. Do whatever it takes, make your heals as huge as possible, with Valithria beaconed and healing yourself from nearby. If all of the suppressors are cleaned up and you had a decent number of stacks up on you, the encounter should be over after about 20 seconds of the burn. Don't forget to toss in Lay on Hands as well, since it will be quite potent. You can also have a Holy Priest cast Guardian Spirit on Val during this time, to boost everyone's healing on the boss.
Seeing 100k Holy Lights is not that uncommon, and with you spamming HL as fast as you can push the button, you can see why holy paladins are uniquely suited to this fight. We've got the most powerful single target heal without a cooldown, and it scales incredibly well with gear due to its slow base cast time. As long as you focus on maintaining your Emerald Vigor stacks and using the correct Beacon / Glyph of Holy Light strategy, you shouldn't run into any problems getting your job done. The rest of the raid, on the other hand, may need some work.
The Light and How to Swing It (Holy Edition) is dedicated to helping holy paladins become the powerful healers that we're destined to be. If you're new to the paladin's healing ways, you can learn the ropes with our Holy 101 article. We also have information on how to keep a tank alive, or how to heal a raid when necessary. Tanking is a job, DPS is a craft, but healing is truly an art.