I love to look at graphs. Well actually I hate graphs, but I love them when I am part of them and they show something that I understand and can relate to, like a 25man LK kill.
I joined a guildrun with Love and a friend of mine (not our guild though), and we burned through 25man ICC pretty easily. Considering the 30% buff, that isn't especially difficult anymore, but it is still interesting to see what was done and how. Or at least I think so. So let's look at the graph from that LK kill and see if we can draw some conclusions about the way healing was done and how it should be done.
First off, healing done for the fight (not including non-healers). I was a disc priest and the other priest was holy. It shows how well disc works for this kind of fight. Alot of aoe damage everywhere and some nuke healing here and there. Shields are just lovely for that kind of damage and especially for this fight since it takes care of Infest.
This other graph might look cluttered at first. But it tells us alot of things. We've got two spikes (except for my last one before the "death". The first spike shows the two shamans and the druid basically doubling their healing output for a short period of time about 2 minutes into the fights. The spike is for about 30 seconds and I am guessing this might be about when the phase transition comes. The Lich King does alot of AoE then that the shaman/druid are raid healing up. Placing yourself close to the edge as a shaman/paladin might be wise since as you can see, the paladins healing drops instead, probably because he has to move.
I'm spiking just before that, which might be due to a infest going off on my shields. Just after the spike we all drop down low except for the holy priest that spikes. This might be us running back into the middle, and he uses some on the run healing like renew and CoH.
In the second phase we all heal about as much as the other except for one of the shamans dropping down low. The only reason I can see for that happening is him running around too much, not being able to find a good spot from where to heal.
Just before the second phase transition we see the same thing, the "mobile" healers spike up for the aoe healing (druid and priest) while the paladin drops really low because he's running out. This time we can also see that the one shaman has been able to get to the edge faster than the other and does more healing during the transition. My healing drops somewhat when the druid and other priest healing spikes, mostly probably because they take care of business (I'll explain what I mean about that shortly*).
Just at the beginning of phase three the other priests healing done drops to zero for 10 seconds. The only explanation I can see for this, since he didn't die, is that he was pulled into Frostmourne and lost from the tracker.
The blue bar represents bloodlust, and here we can see something interesting. The druid dies, but what happens with the paladin? Instead of increasing healing done during bloodlust, she dips significantly. It is also interesting to see that none of us healers spike during this. As a disc healer you don't benefit much from Bloodlust, but the shamans and paladins should. Instead, their healing is lower than ever. My guess here would be that the shamans are starting to oom. The paladin is probably stuck behind a defile and had to run most of the Bloodlust duration (this is only guessing though).
The first red stripe during Bloodlust is the death of one of the tanks. See how this coincides perfectly with the paladin heal drop? Not only the paladin, just before we can see the other priests drop and the druid is dwindling down too. The druid dies at the same time, and they both die of Spirit Bursts. These heal drops are surely the reason to why the tank dies (he got a combat res and we made it anyway fortunately).
The second stripe represents the resurrection of the tank (as I just mentioned), and we can see how the paladins healing spikes just after that. 1,5 min later, the paladin dies (Harvest Soul) and every healers healing is slowly going down except mine that spikes again, maybe as a counter act on the other healers healing dropping. 10 minutes into the fight we all die, whichh is part of the event, and the fight is basically over from there.
Now let's look at what heals were cast.
The uptime of Shield and Aegis should preferrably be as low as possible because the lower the uptime, the more likely that it was fully absorbed. About 70% of my healing is just absorbs through Shield and Aegis, and that is what disc priest is all about. Like I have said many times before, absorbtion offers a unique way of healing thorugh damage reduction, and it should really be used to the max. Don't waste mana on Penance and Flash Heals unless you deem it really necessary. Otherwise you must trust your fellow healers to do their job properly and get the sniper healing done.
In a fight like LK where there is aoe damage everywhere and you can expect anyone to take damage at any time basically, there is no reason not to keep shields up on people at all times and also to use a Prayer of Mending as often as you can.