Horridon is indeed fun to heal as a disc priest though the thought of solo healing it is a little scary to me. I have heard of priests who reforge and gem completely for haste specifically for that fight, and that is probably how this is done.
Megaera I find fun as well. Our first few kills of this fight I actually forged completely over to Mastery to get it down. The big trick to this encounter is to use Spirit Shell every time you stack for a new head. Another useful thing is ensure that your melee is in one group and your ranged is in another since most groups will be loosely clumped into those groups already, making your prayer of healing more likely to hit everyone in your group. Beyond that, try to save your mana for the end and avoid using Atonement as much as possible for the last two heads.
The one fight I have trouble with as disc is Primordius, and that is because in order to atonement heal effectively you need to be fully evolved. This means that you either spend time gathering stacks, or you just don't use atonement. This is the fight that made me decide to go dual spec holy/disc, though I don't think you have that option. My advice to you for this encounter is to just forget about evolving and don't use Atonement as it is a waste. Instead, focus on single target and then use your spirit shell when raid damage is going out. The tanks take enough damage during this encounter for this to be viable.
I've created a number of macros and listed a few addons that I use when healing as disc and I find these to be very valuable.
A lot of people suggest using a healer addon such as VuhDo, Grid, or HealBot but I find the default UI with Clique is just perfect and while the healers in my raid are fiddling with their filters trying to get this debuff to show or that one not to show the default UI already shows everything that is important to know.
Below are some macros I have found useful as well as a list of addons I use to heal.
To make Cascade always function as a heal no matter what you're targeting:
/cast [target=mouseover,help] [target=target, help] [target=targettarget] Cascade
The inverse of the above macro, to make Shadowfiend, Holy Fire, Smite, and Penance always function as a damage spell and target the boss while you are targeting the tank. This is especially useful for fights where you are Atonement healing and there are a lot of target switches. Just change Penance to the spell you want.
/cast [target=mouseover,harm] [target=target, harm] [target=targettarget] Penance
Super Spirit Shell - for when you know a huge chunk of damage is coming and you need the biggest spirit shell you can get. You can attach engineering gloves and Lifeblood to this as well if you want.
#showtooltip Spirit Shell
/cast Spirit Shell
/cast Power Infusion
Leap of Faith - because you don't necessarily have time to target the person you need to cast leap of faith on, or maybe you don't have time to see the name of the person you need to grab. This lets you cast Leap of Faith on whatever you have your mouse on, be it a raid frame or the physical character. It adds in a confession for good measure, because 90% of the time I use LOF to save some dumbass standing in the fire.
#showtooltip Leap of Faith
/cast [target=mouseover] Leap of Faith
/cast [target=mouseover] Confession
This is the only macro I feel I need for Shadow, and it works quite well with all three specs. Just plug this onto any spell you want to use but not have it break your Mind Flay, Penance, Mind Sear, or Hymn. This lets you spam the button without breaking any channeled spell.
#showtooltip Mind Flay
/cast Mind Flay
As for addons, there are quite a few I highly recommend.
Clique: this works well with the default raid UI and allows for click casting of your spells. A must if you want to be a strong healer.
Inline Aura: Gives you more detailed information about what's going on with your spells inside their action buttons.
Quartz: A very useful cast bar replacement.
Ingela's Rapture: Helps you keep track of your Rapture procs to maximize mana returns as disc.
HaloPro: A range indicator to help you maximize the effect of Halo.
Weak Auras: I consider this an essential addon for any class or spec. You can configure this to show you anything you want, really. I have bouncing icons telling me when important things like Solace, Shadowfiend, and Prayer of Mending are off cooldown.
Leatrix Plus: A tiny addon with a load of excellent tweaks to the UI that you can turn on or off as needed. Though not necessarily needed for priests specifically, it's a largely unknown addon that adds too much functionality to be ignored. This one as far as I know is only available from wowinterface and not curse.
Edited by Purl on 4/16/2013 8:43 AM PDT