Welcome to the end boss. It is possible to heal through Arthas with a less than ideal composition -- my first kill with 25 players was done without the presence of any holy paladins. If your guild is still struggling with it and the issue might be from the healing side of things, maybe I can help you out here with some additional pointers. If you're not quite sure about the overall mechanics of the encounter, refer to Kinasthesia's awesome tutorial videos: Part 1 and Part 2.
I hope you're familiar with the general encounter mechanics and the strategy as outlined. I'm just going to stick to addressing any aspects that will affect you directly as the healer.
Phase 1Put one healer on each tank, and the rest can raid heal. You can float a third healer on both tanks.
Infest You'll be facing this spell for most of the encounter. Personally, I chose to have that covered by a discipline priest and a holy priest. A discipline priest gives you an extra edge by preemptively casting Power Word: Shield on specific players. If possible, assign specific groups for each priest to cover. Any players who stand in close proximity to each other for most of the encounter (such as a melee group or a healer group) should be assigned to the holy priest. Once everything is assigned, what will happen is Arthas will cast Infest, and players who have been shielded will see it fall off. Any remaining players with Infest can be rapidly covered by that holy priest, between Circle of Healing and Prayer of Healing.
With other healing classes, rapid-firing Chain Heal from a shaman or Wild Growth can just as easily cover the same mileage.
Use any healing spell at your disposal to clear Infest off your players.
Necrotic Plague Designate one player to dispel this debuff off of the affected player. If you happen to have a priest with Body and Soul, it can help out significantly (especially if your guild just started working on Arthas). Call out the name of the player who is affected by Necrotic Plague. Once they're close enough to the group of Drudge Ghouls and Shambling Horrors, cleanse the disease off of them immediately and them tell them it's safe to run back. Ideally, you don't want an Infest healer doing this. Lastly, make sure plagued players don't run in front of a Shambling Horror.
First transition phaseSet up two positions where healers can go and stand. Be sure to divide them up accordingly. For instance, I like to position a holy paladin, a resto shaman and a discipline priest on the side close to the throne. I'll then have another priest and two druids on the side that is closer to the teleporter.
Raging Spirits aren't fun. If you happen to get cloned with a Raging Spirit, stop what you're doing and try to run toward a tank. There have been cases where a Raging Spirit will just destroy a healer or any player who is standing next to them. That would be proximity aggro at work, I suppose.
Tanks will periodically get silenced with Soul Shriek. Make sure that debuff is removed immediately. Otherwise, the tanks may not be able to use their abilities to pick up aggro.
Once that third Raging Spirit comes up, you'll know it'll be time to run to the center. But be careful when running back in! Ice Spheres are not fun to run into. Your fellow guildies might make fun of you for it. Trust me, I know from personal experience.
Phase 2Now that we're all back on the central platform, this is where the real fun begins.
Soul Reaper This is an instant spell and has the potential to destroy tanks. If he's geared and lucky, it is possible for a single tank to tackle Arthas all on his own. But that tank will need your help to pull it off. I don't recommend doing it that way, but it is possible to do. Keep your defensive cooldowns close and ready to go at a moment's notice.
Infest Wait a minute! Didn't we just talk about this in phase 1? Yeah, we did. To cover a ticking debuff like Infest when there isn't much stuff going on isn't a big challenge. When you start factoring in Defile and the flying death angels, that's when Infest management becomes a problem. At this point, you may wish to pull a third healer on board to help you manage the Infest load. The times that I've had trouble dealing with are when Defile and Infest kick in at the same time. Players end up going all over the place or can end up out of range. Having that third set of heals helped enormously in stabilizing the group during this delicate phase.
If you happen to get snatched up by a val'kyr, don't panic. Simply call out that you've been grabbed and that you were either tank healing or Infest healing. It is absolutely crucial that the rest of the healers know where to pick up the slack until you are freed from their grasp. Using a raid-wide defensive cooldown can help buy you extra time when any Infest healers happen to get picked up with an Infest active on the raid.
Second transition phaseThis phase ends up being the same part as the first transition phase. The difference is that you have an extra Raging Spirit to deal with now. Feel free to help chip in with the DPS if think you can get away with it. Healing isn't that stressful during this stage.
Phase 3We still have Defiles and Infest to worry about. Without the threat of val'kyr, though, they're just not as fatal an issue.
Harvest Soul If you have customized raid frames, make sure this debuff shows up on the list. Any player who gets targeted by this needs to have it healed right away. If they're alive after six seconds, they'll get sent down into Frostmourne. Any raid healers who aren't assigned to things like Infest or tank healing should keep a very close eye on this.
Did you get sent inside Frostmourne? All you need to do is heal Terenas. If you can interrupt the Spirit Warden when he's channeling Soul Rip on the king, you should do it. Otherwise, try to remove Soul Rip by dispelling it since it is magic. By the way, since Terenas has Light's Favor, unload your biggest healing spells on him. The more health he has, the more damage he'll unleash. Once the Warden is taken care of, you'll be knocked back up to the surface.
Once you reach the final 10%, Arthas will destroy your raid. Feel free to start nuking him with your assorted damage spells once you get resurrected. It does end up being a form of stress relief.
Good luck in getting the Lich King down! Have any additional class specific healing tips to add? Or any healing tips in general? Add them in the comments!
Want some more advice for working with the healers in your guild? Raid Rx has you covered with all there is to know! Need raid or guild healing advice? Email me at firstname.lastname@example.org and you could see a future post addressing your question. Looking for less healer-centric raiding advice? Take a look at our raiding column Ready Check.