Thanks Memoryz for all the suggestions. We run this with our normal healing team, 2 palis, 1 disc priest, 2 holy priests (I'm one of them), a druid and a shaman. The disc priest we have is great and he is usually on the Maloriak tank managing the engulfing darkness mechanic. I generally only go disc on Heroic Halfus and Heroic Atramedes. We got to the first red phase a few times and quickly realized the need for a raid cooldown rotation for the scorching breath, we are using Barrier, DG, and AM. I think the plan is to try to save tranq and hymns for the final phase so it sounds like we are on the right path.
We did assign groups to heal for the holy priests during the Red phase and that seems to help but it definitely looks like raid cooldowns are required in addition to massive amounts of burst healing.
Our biggest problem right now it seems is the ominous sludge and interrupting all those storms. Heck, sometimes we haven't been good at spell stealing Remedy, but we will get there. It is funny how on normal mode you can just hammer the encounter with enough DPS and HPS and still get it done, but on heroic mode the simple mistakes wipe the raid.
P.S. I'm wondering if having a second disc priest for this fight might be a good idea.