As Holy on the Chimaeron fight, I have considered using a glyphed Holy Nova for healing Caustic slime, as unless it is on me, I usually find myself desperately spamming flash heal to cover it, which is eating up my mana. The hope is, with Chakra: Sanctuary, Mastery and Test of Faith, I can clear the 10k in two to three casts. The issues I see is keeping people in range, and the way HN has DR on heals.

If this isn't viable, what can I do to improve myself on this fight? Flash Heal spam is eating my mana up, plus a little lag on the 2nd heal can be disasterous. If I get slimed, I use Binding Heal, but otherwise I feel as if I am underperforming.

I'm pretty sure the sim will not SW:D above 25% with the default action list. The DPET you're seeing is only for SW:D below 25% only.

Can't afford to stand close enough together for Holy Nova, or Caustic Slime will hit multiple targets. This is a non-continuous casting fight, and should be approached differently than most encounters. I'd recommend adding a debuff marker to Grid for "Low Health", which will show when a player is below 10k. Also for this fight, I switch the Health Deficit display (on CenterText2) to Unit Health. Often there will be enough passive healing (Echoes, VE, totems, warlock) for someone to reach 10k before the next Slime is cast. After Massacre, there is plenty of time to cast slower heals, the key is not to overdo it (raid needs no more than 10k each to survive). Circle of Healing is very strong now as well.

We have raid healers focusing on their group first before helping on other groups to avoid some overlap (25-man normal). Also for tank healing, only the double-hit tank needs to be healed to full, the main tank will survive as long as 10k is reached between hits. Here is last night:

World of Logs - Real Time Raid Analysis

This might be the solution I am looking for. As a ten man, we split the groups up, and when I get two spit victims in my group, there's usually zero margin for error when Flashing. A PoH does seem the better option.

Hopefully that will save some mana here as well. Granted, it's only our second week on trying the encounter, but it eats my mana up.

I do this fight in 10m as well. I find the best strategy is going Serenity and using Holy World: Serenity -> Flash Heal when you have two Low Healh on your same group. Also keep in mind that 25% of those situations you will be one of the people targeted as well, in which case a Binding Heal is the perfect solution. If by some chance you get two sets of slime on your group in a row and Serenity is on CD, two Flash Heals will hit fast enough and is more consistent than PoH which doesn't hit for 10k anymore. Serenity and binding Heal will save you so much mana that in these situations an extra Flash Heal won't kill you.

Actually, I added in the following line just above the Mind Blast line:

actions+=/shadow_word_death,if=mana_pct<50

I'll admit I'm a bit new to SimC so it's not beyond a doubt that I could have messed something up. Kudos to Alinna for this line suggestion which can be found in the Shadow Cataclysm thread. Also, of course, the percentage can be changed, but on live I personally try to keep SWD on CD sub 50% mana to ensure I can avoid using Dispersion as much as possible. I believe we can all agree that in most environments using SWD is much better than casting Dispersion, considering Disp. is a direct damage loss due to inactivity. Assuming our GCD is at 1s, that would mean SWD gives 10% mana while Disp gains only 6% in the same amount of time. Granted, this doesn't take in the CD on SWD, but if you're looking for a quick mana gain, the choice is clear, I'd say, especially if you're just trying to keep your mana on "Life Tap Support" while waiting for the last few seconds of AA/Sfiend to come off CD.

According to sims I've run with and without the aforementioned line, there was only a 4 DPS loss with the line, but I'd appreciate someone who is more experienced with SimC to double-check my conclusion here.

With the nerf, a PoH is not a safe option between caustic slime casts. After adding in mastery/glyph it will get people to 10k, but the caustic slimes come every ~4s, meaning you don't have time to wait the 6s it takes those hots to tick. Heal is actually sufficient, however; it hits for about 9.5k for me, combined with a tick or two of Healing Stream or VE and a tick of mastery hot, it gets the job done, as long as you are reasonably quick. I reserve flash heal for times when I'm in the middle of a gheal on tank or something when the slime hits.

For my 10man, rather than dividing the groups and risking a single healer having to cover two people, we divide up the targets the same way we used to do with Penetrating Cold. We simply number the raid members (it helps that the healer I do this with and I have our grid sorted the same way), and he gets the lower slime target while I get the higher one. This leaves our third healer on tanks full time, and I still cover both groups after massacre via a pair of PoH, when you have plenty of time to allow mastery/glyph hots to tick before the slimes start up again.

As a note to unholy_prince's comment, the first couple of times I was in there I tried doing the easy phases in Serenity as HW:Serenity is perfect for slime targets, but I decided not to bother after back-to-back feuds left me stuck in Serenity for a feud phase. Additionally, you now need to be in Sanctuary for a PoH to get a group over 10% after massacre, so being in Serenity at all is dangerous if you are responsible for getting a group to safety after massacre.

Eh it's not hard to not refresh Serenity when you know a Feud's about to come, you lose maybe 1-2 cycles where you have to throw an extra Flash Heal.

You have ~15 seconds after a Massacre to get people topped off, even if your HPal isn't popping Holy Radiance after each one (CD lines up) you have other effects like Healing Stream which will be more than enough.

How come I see so many high end priests reforging mastery into haste, even when above 12.5%?

Since feud can happen after consecutive massacres, and massacres come every 30s, entering Serenity even immediately after the massacre following a feud can result in being in Serenity with Chakra on cooldown as he begins to feud (though you might mostly avoid this problem with 2/2 in the new state of mind, something I haven't tried). Considering feud is the healing-intensive phase of the fight, it seems silly to me to potentially damage your ability to get the raid up during that time just for a bit of convenience and minor mana savings during the lulls.

As for standard post-massacre topping, obviously additional healing from holy radiance/healing stream/whatever can cover an underpowered prayer of healing, though the holy pally I run with doesn't need to waste mana on holy radiance outside of feuds because he knows I'll have the raid into safety without help; requiring any other healers to waste mana when post-massacre healing can be covered by exactly one PoH per group seems silly. All that said, this is really academic for me, as I haven't started chim on heroic, and the normal mode is pretty generously tuned in terms of actual healing needed.

Haste is the best throughput stat if you don't have mana problems.