I always have a tough time deciding how to compose my healing guides for raid bosses. Talking about holy paladin theory is fun, but at the end of the day, we're actually being thrown into real raid encounters. We're expected to know how to heal through the incoming damage and handle all of the boss' special abilities. Knowing the specific healing per second of Divine Light isn't going to help you keep your tank alive. At the same time, there's no point in turning the holy paladin column into an esoteric version of StratFu by giving you a play-by-play review of every boss encounter.
Instead of boring you with a list of boss abilities that your raid leader is going to repeat anyway, I am going to try breaking down only what's specifically important for a holy paladin to know. You'll want to know when it's safe to use Divine Plea, when Holy Radiance is going to be most effective, and what you're supposed to dispel. Who should you put Beacon of Light on? Are any of our "Hand of" spells worthwhile on this fight? In addition, please feel free to ask any additional questions that you'd like to see answers to or add any suggestions.
The first thing you'll find out about Halfus Wyrmbreaker is that you're going to treat the encounter differently every single week. Halfus gains new abilities based on the drakes that are active on any given week, and these abilities dictate how we'll be healing. The Orphaned Emerald Whelps and the Time Warden both grant Halfus' pet Proto-Behemoth abilities that will deal widespread raid damage, while the Nether Scion and Slate Dragon will make healing the tank quite difficult.
Everything goes uphill from here
Regardless of the week's drake combination, Halfus is a front-loaded fight for healers. At the start of the encounter, you'll be healing at least two tanks, and they'll be dealing with Halfus and a few of the responsive drakes. The drakes, while seemingly innocent, actually deal a respectable amount of damage to tanks, and so you'll be pulling out all of the stops to keep your tanks alive at the start of the fight. If you have an AoE-heavy drake composition, make sure to use Holy Radiance during this early phase while keeping the tanks alive. Aura Mastery can also be useful to mitigate the AoE fire damage, so use it early.
Don't mess around with trying to use Holy Light too much until you've killed the drakes, as the incoming damage is simply too high. Divine Light is the right heal for the job, and you should use your regeneration abilities or trinkets early to sustain your throughput. Don't hesitate to pop Avenging Wrath, Divine Favor, or your Guardian of the Ancient Kings -- this is literally the perfect use case for these cooldowns. I prefer putting Beacon of Light on the main tank and then using Tower of Radiance procs from my Divine Lights to grant me enough holy power for a free Word of Glory. I also use my Hand of Sacrifice to absorb some of the tank damage, especially if the Slate Dragon or Nether Drake is around. As the drakes start dying, you'll notice that the incoming damage will drop dramatically. As soon as you've finished off the drakes, use your Divine Plea during the lull before phase 2.
The rest of the fight is relatively simple to heal; just make sure you keep the tank topped off, as you won't be able to heal them during the periodic stuns. If the Slate Dragon was active, you can also use your Hand of Protection to clear the Mortal Strike debuffs from your tank, allowing them to receive the full benefit of your healing spells. Holy Radiance is a simple way to heal everyone up after a Furious Roar, though you might have to avoid stacking too closely if the Proto-Behemoth is shooting fireballs your way.
The twin dragons are a perfect example of why healing meters can lie about what's really happening. When your guild first starts working on this encounter, your incoming damage is going to be massive. Once everyone learns their roles and positions, you can get through the fight without taking much damage at all, which in turn makes it look like we're doing less healing than usual. Our goal as holy paladins is to keep everyone alive long enough to learn the fight, with the hope that they'll learn it well enough to give us time to breathe.
When dealing with the Meteor-like spell Blackout, you'll want to pop Aura Mastery right before you dispel the debuff. The spell deals shadow damage, and so Aura Mastery will knock a ton of damage off the total hit. Make sure that you don't dispel it until after there are a few people in range of the debuffed player; otherwise, you might kill someone. It's sort of like dispelling Necrotic Plague, except the player with Blackout moves toward your raid, which I guess means that your raiders are actually Drudge Ghouls.
I like to use Holy Radiance to clean up the damage dealt from Blackout, though don't stack too closely after a Blackout (you're avoiding Theralion's orbital bombardment). The tank damage should be fairly light, so toss them a Beacon and focus on topping off your raid. Don't waste mana on Divine Light unless you need to, as Holy Light and Holy Shock are usually enough to get the job done. You're basically saving your mana for the more healing-intensive Theralion phase, so be conservative and use small heals. You can use Divine Plea during either of the phase transitions, as there's no abilities active and generally not any healing needed.
AoE healing handles Theralion's threats
Holy Radiance is basically the ultimate spell for countering Theralion's abilities. You'll be stacked up with the rest of the ranged classes, and Holy Radiance is going to be super-effective. Not only does it provide powerful healing to those nearby, it will also give you a speed boost when used, allowing you to quickly get out of the void zones that Theralion spits out. I like to use my Divine Favor and then pop a Holy Radiance if my raid gets particularly low on life, as the extra haste from DF will give HR a few bonus ticks of healing.
The only catch to using Holy Radiance is that you want to wait for Theralion to use his Engulfing Magic before you pop HR. Engulfing Magic makes you nuke everyone nearby when you do any healing, and if HR is active and you get hit with EM, you've just killed everyone. As soon as you see EM get cast on someone else, you're safe to use HR. If you do get hit with EM, you'll do double healing while it's active, so use that time to heal everyone up while standing away from them. Make sure to keep an eye on the tank during this phase; since you won't be hard-casting many heals, they won't see many Beacon heals.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Learn the ropes in Cataclysm 101 for holy paladins, study the new balance between intellect and spirit and learn how to level your new Sunwalker. Tanking is a job, DPS is a craft -- but healing is truly an art.