The three mini bosses in Ruby Sanctum aren't much to worry about, to be honest. Just like the three dragons in Obsidian Sanctum, surviving the brief encounters only requires that you know what to expect.
Baltharus the Warborn As a healer all you need to do is hang back and spam heals on the tank. What you heal with doesn't matter too much, just expect the damage to be a bit spiky. There is a knock back ability Baltharus does, but you should be at range so it won't matter too much. At 66% and 33% (50% on 10-man) Baltharus will clone himself and the clones will need to get picked up by the off-tanks, and at that point you'll need to start healing the off-tanks too. Again, it doesn't matter how. Baltharus has an attack called Blade Tempest where he spins his weapon around and does a pretty hefty sum of damage to your tanks (and any DPS standing in front of the boss.) It will get worse if any players with the Enervating Brand debuff are in range of the boss, which they may well be if you're in a pug, so be ready to toss out a priest cooldown if things look dire.
Of the three mini-bosses, I would say this one is the hardest, so if you get through Baltharus without batting an eye (it's quite possible to just muscle through the encounter without properly handling the debuff) then you shouldn't have any trouble with the other two.
Saviana Ragefire This mini-boss is so easy you'll forget you fought her. She spends a good chunk of time up in the air spitting fireballs at you. She'll mark members of your raid with an arrow to cast Conflagarate on them. These players will need to run out of the raid and all you need to do is throw them a few heals. Beyond that the damage on the tanks and raid is forgettable. You could practice your Smite spamming here if you want, especially if your DPS are melee heavy.
General Zarithrian There is some tank swapping required to handle his sunder armor attack, but it's nothing too strenuous. The major concern you'll have is the adds, which come from the left and right of where Zarithrian stands. Stand close to the boss so it's easy for your off-tank (or whoever isn't swapped in) to pick them up. Keep Fade ready to go in case an add spawn lines up too closely to an Intimidating Roar. Intimidating Roar will incapacitate you with fear for a moment, but it doesn't last long enough to devastate your tanks, nor even constitute me trying out Fear Ward.
The big baddie of Ruby Sanctum is an adorable pink dragon named Halion. The mechanics for the fight are pretty straight forward, but your raid must be coordinated and aware to succeed. Basic dragon rules apply to Halion: don't stand in front of him, don't stand behind him. You can get breathed on or cleaved in front, and from behind you'll get stunned by Halion's tail. The stun doesn't seem so bad, but depending on what part of the fight you're on, it could be lethal.
Your Role As a priest you'll find one of the best things you can do this fight is help where help is needed. Your raid will be taking a fair share of damage, and so will the tanks. So, regardless of what your role is, you'll be raid healing and tank healing in this fight. This means it's mostly up to you which spec you'd rather play -- do whatever feels right. Holy does have an advantage with Body & Soul though, (more on this later.) Discipline's bubble spamming isn't as all-powerful in this fight either, just because the damage occurs off and on. This means you'll want to focus a lot on single target tank and raid healing instead.
The encounter is split up into three phases, a top phase, a bottom phase, and a split phase.
Phase 1 In this phase, the major concerns for healers (and everyone, really) are Meteor Strikes and Fiery Combustion. Meteor Strike will mark a location on the ground (typical void zone mechanic) and you will need to dodge away from the area to avoid the impact. When the meteor lands, arms of fire will spread outward and players will also need to avoid standing in this fire. Simple stuff, really. On heroic mode, the Meteor Strikes will also spawn big and little adds which need to be tanked. Have Fade ready when they show up.
Fiery Combustion is similar to Grobbulus' clouds in Naxxramas: when you are afflicted you need to move to a safe, designated area and let the debuff tick off or be dispelled from you. When the debuff is removed (one way or another) a big circle of fiery doom will instantly form under you and cause damage to you and anyone else standing in it. Get out quickly! (It does have a knock back ability, so this shouldn't be too hard.) The size of the circle will depend on how long it takes for the debuff to be dispelled.
Any player with a friendly dispel ability can remove the debuff (it is categorized as both a curse and a magical debuff) but it's best if just one player is in charge of it, just to avoid early or sloppy dispels.
As a priest you'll want to be spamming heals on the tanks, and after a Meteor Strike, the raid as well. Holy is strong to react, but if you want to apply some preemptive bubbles you'll be best selecting targets off your screen based on how close they are to the impact area of the Meteor Strike. If you are on dispelling duty, don't use Dispel Magic until you are sure that your target is in a safe location. Don't just count off and expect them to be where they should. Look at your screen and make sure you know where they are when you dispel the debuff. If you aren't dispelling, make sure to spam heals on the target with the debuff so they stay alive.
Body and Soul comes into play by helping players with Fiery Combustion move to their designated position faster. All you have to do to utilize it is as soon as you see the debuff on a player, cast Power Word: Shield on him. (Just remember that if you're running with a disc priest to make sure you both discuss shield usage.)
Phase 2 In the second phase your raid will take a portal down to the twilight realm. Whatever you do, DO NOT go into the portal until the tank has gone in first. There is a big scary dragon on the other end of that portal and he will aggro onto whoever shows up first; this is why the tank goes first. Because he'll momentarily be without heals though, make sure to toss him a Guardian Spirit or Pain Suppression before he goes through the portal so he has some extra survivability while everyone is catching up.
Phase 2 has something called Soul Consumption, which acts nearly identical to Fiery Combustion, but instead of fire damage, it deals shadow damage. Additionally, instead of a knock back, the zone will now slows players caught inside it while deaing damage. This wouldn't be terrible, except another mechanic exists in phase 2: Twilight Cutters.
Twilight Cutters form when 2 (or 4 on heroic) shadowy balls rotating around the room connect. This occurs periodically, and when it does the cutters will become instant death lasers. You will instantly die if a cutter connects on top of your, you move into one, or if you touch the outer wall when the cutter is active. So now imagine the slowing debuff from Soul Consumption (as well as the tail swipe stun from Halion) and you can see phase 2 is a bit tricky in comparison to the first. Healers will need to be sure that where they dispel a target isn't soon going to turn into a dangerous spot.
Oddly, heroic is actually a little easier in regards to the cutters. In one respect you have to be more on your toes about avoiding 2 death beams. On the other hand, having 4 floating orbs means you always have a point of reference on your screen to know where the cutter will form. If you're doing normal, make sure you always have your camera pivoted so you can see at least one shadow orb.
As a priest, keep rocking tank and raid heals. There is ticking shadow damage in phase 2 that you can respond to with Renew or Power Word: Shield spamming; just don't overwork yourself if you're bubble spamming, since you don't want to prevent a Body and Soul from taking effect.
Phase 3 The third phase is a mix of both phase 1 and 2. You will have to split your entire raid up and DPS Halion in both realms (while dealing with the mechanics from the first and second phases.) As long as the DPS is even, the damage should remain similar in both realms, meaning that even if you split up your healers, there shouldn't be any surprises.
In heroic mode, your raid must place Combustion and Consumption zones carefully because the zones in one realm will also appear in the other.
As a priest you'll want to continue with the appropriate phase routine, based on what realm you're in. Keep using Body and Soul if you're a holy priest and don't forget that no matter what your job is, you need to watch the tanks. If you stay calm, a kill show be no trouble.
Kinaesthesia's tip for heroic Halion: If you're a daring holy priest healing up top and happen to have a paladin or death knight tank, try helping out with adds by using healing aggro to your advantage. Move into the AoE of your tank's Consecration or Death and Decay, and then as the adds spawn, throw out a Circle of Healing. Circle of Healing produces a lot of threat, so it should direct all the adds to you and then, thanks to your genius position, to the tank. If you do this instead of using Fade, you'll be helping out any of your fellow healers who don't have threat drops.
Below I've picked out the drops that may interest you as a healing priest.
- Abduction's Cover (heroic version) -- 10-man -- This is like a direct upgrade from the Heartsick Mender's Cloak off 10-man Blood Prince Council in Icecrown Citadel. It's not much for min/maxing, as MP5 and haste are a strange combo for priests (spirit and haste for holy priests, or crit and MP5 for disc priests would be more sensible if you're looking for a cloak with regen.) Still, if your cloak is lacking, you'll definitely be able to utilize the stats on this, regardless of what healing spec you sport.
- Cloak of Burning Dusk (heroic version) -- 25-man -- If you rate item value on spellpower (which I do) this will be your best-in-slot cloak, whether you do or don't do heroics. With an item level of 271, and 284 on heroic, this cloak will be better (respectively) than the previous best-in-slot: Frostbinder's Shredded Cape from Valithria Dreamwalker. There is a slight trade between crit and haste, but it's not going to break anything if you choose between them. Plus, the red gem slot and spellpower bonus make the cloak extra appealing.
- Glowing Twilight Scale (heroic version) -- 25-man -- The spellpower alone on this trinket is quite nice, but I'm sure many healers will be turned off by the on-use effect over a proc. The range conditional on the group healing effect makes the trinket a little more ideal for a tank healer (melee would always be in range of your target), but if you're a raid healer don't give up on it: there is nothing to keep you from using the trinket to focus more heals on clusters of players when there is heavy raid damage.
Binding the use effect will probably lead to overhealing, so in that regard the effect is a bit clunky in comparison to a spellpower buff. Still, you might want to nab it and test it if you don't have anything better. I still prefer the smart heal of the Althor's Abacus myself.
- Bracers of Fiery Night (heroic version) -- 25-man -- Hello best-in-slot bracers. Dawn pats the bracers affectionately. As you may recall, there were not pure throughput bracers in ICC, just two pairs with spirit on them. Now Blizzard has graced us with a nice pair of crit and haste bracers for priests and clothie casters to fight over. Good luck!
Want to find more great tips for carrying out your priestly duties? Spiritual Guidance has you covered with all there is to know. Check out Holy 101 or Disc 101 for an introduction to healing as a priest, or for the party-minded healer, check out a priest's guide to tanks.