As we delve deeper into the Bastion of Twilight, each boss is more powerful than the last. While Halfus Wymbreaker and the dragon twins aren't the easiest raid bosses by far, the Ascendant Council and Cho'gall are on another level of difficulty. They're not only more complicated, but simply more punishing as well. You're going to use your full spectrum of abilities on these fights.
The Ascendant Council encounter emphasizes the importance of good positioning and healing on the move, while Cho'gall will stress both your AoE and single-target healing throughout the fight. Holy paladins have several abilities that are uniquely suited to each encounter, and knowing how to use those spells will be crucial to healing effectively.
While fighting the four elemental lords that Cho'gall has rounded up, you'll first be faced with two of them at a time. Once you've dispatched each pair, the four combine their powers (à la Captain Planet) to form the powerful Elementium Monstronsity. Your goal is to save as much mana and as many cooldowns as possible for the frantic burn phase while facing the Elementium Monstronsity, as this is where the incoming damage will be the most intense.
Ignacious and Feludius
The first elemental pair represents fire and water, and your major focus will be healing through Ignacious' Rising Flames attack. I like to use Aura Mastery to help mitigate the raid-wide fire damage from the first, and then follow up with Holy Radiance after each to top everyone back off. You can toss Beacon on one tank and then heal the other, as there's not going to be a ton of tank damage being done. Try to conserve as much mana as possible, using Holy Shock and Holy Light to do the majority of your raid healing.
Your guild may do things differently, but we typically dispel the pair of magical debuffs that the bosses will toss out periodically. Your DPS players can switch targets based on their debuffs to a particular boss, but I just use Cleanse to get rid of them immediately. Ask your raid leader for guidance. There's also a Waterlogged debuff that Feludius will apply with Water Bomb, which is similar to the acid debuff from Acidmaw in Trial of the Crusader. Just run through some of Ignacious' fire to get rid of it.
Arion and Terrastra
The second elemental pair represents air and earth, and your major focus will be obtaining the proper debuffs to avoid each elemental's super-ability. If you're familiar with the Twin Val'kyr fight of Trial of the Crusader, this is very similar. When Arion's using his powerful Thundershock, you run into a gravity well to become Grounded. When Terrastra is using his powerful Quake ability, you hop into a vortex to become airborne. You'll be running around a bit to avoid being hit by a tornado or gravity well at the wrong time, as they do move around the map.
If you see one of your raid members has failed to switch to the proper debuff, be aware that he's going to get hit for a ton of damage once the super-ability is finished. Plan to use a few Divine Lights to top him off quickly.
The Elementium Monstrosity
As soon as your DPS is ready to finish off Arion and Terrastra, you should use Divine Plea. There's a short intermission phase where everyone is stunned and not taking damage, which means you can regenerate a ton of mana without worrying about the healing penalty.
Once the boss begins attacking, get ready for some serious healing. There's a lightning attack that randomly targets and chains off of players, and so you'll see a lot of raid damage done. The pools that the boss leaves behind will also begin to restrict your movement options, and as you run out of space in the room, the lightning will be chaining more often. The amount of healing you'll need to do will ramp up, so be sure to utilize all of your cooldowns to help ease that burden. You'll need as much mana as possible coming into phase 3, as you want to give your raid as much time as possible to finish the boss. Don't underheal here; if you fall too far behind, you'll see players start to die.
Cho'gall is by far the most difficult challenge you'll face in the Bastion of Twilight. During the majority of the fight, he'll be absorbing shadow and fire elementals, which imbue him with special powers. During the Shadow's Orders buff, the entire raid will be taking quite a bit of shadow damage. This is the perfect opportunity for you to use Aura Mastery and Holy Radiance. You can also personally use your Divine Protection to cut the damage down, and the Glyph of Divine Protection can be useful on this encounter. Don't use big heals to bring people up from Shadow's Orders, as it only lands a couple of times and then stops, giving you plenty of time to heal everyone up at your own pace.
The Flame's Orders buff will increase the damage that your tank will take, so make sure that his life doesn't dip too low. I'll typically use my Hand of Sacrifice on the tank during this phase, if only to save me some mana since my Protector of the Innocent healing is free. Managing these two abilities will be the majority of your job for this fight, at least until the final phase begins. Your tanks will probably be trading Cho'gall back and forth due to his damage-increasing debuff, Fury of Cho'gall, so simply heal whichever tank has aggro.
Cho'gall has one additional mechanic, which is Converting players to worship him. You can use Rebuke, Hammer of Justice, War Stomp, or Arcane Torrent to interrupt players that are converted. It's worth the extra GCD or mana to interrupt quickly, because if the worshippers aren't interrupted fast enough, the boss will deal additional damage. Be prepared to give your tank or raid some extra healing. I typically use Hand of Sanctuary on the tank any time I see Cho'gall gain more than two stacks of his Twisted Devotion buff.
The race against corruption
While your team shouldn't be getting hit by any of the corruption-causing abilities, they might make mistakes when learning the fight. The most important thing to remember is that when players hit 25 corruption, a magical debuff called Corruption: Accelerated is placed on them that starts causing additional corruption. Dispelling this quickly is very important, so make that your top priority when you see it. In phase 2, everyone gains corruption at a steady rate, so you'll be dispelling this off of everyone at some point.
At the end of the fight, everyone starts deteriorating slowly. You'll be taking continuous damage and your corruption will be building up, so your job is to keep everyone alive through this battle of attrition. If you're targeted by one of the tentacles that pop up in this phase, your healing done will get cut by 75%. Quickly Rebuke or Hammer of Justice the tentacle to bring your healing back up to normal. Remember that Holy Radiance is very potent for healing up your entire raid, so use it often. Just like the Ascendant Council, you'll be using your cooldowns and excess mana to simply keep everyone alive long enough to finish off the last quarter of the boss' life. If you were able to intelligently heal through the elemental Orders earlier in the encounter, you should have enough mana in your tank to finish the job.
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.