Its not your fault entirely. I have been saying this would happen. Check out the other big disc thread here for some more advice on what you can do.
The main thing is stack spirit spirit spirit and then more spirit. You have far too little. All gems slots that don't give mastery and spirit slot bonus should be used for spirit gems Use a spirit flask and food. If you have the hc DS trinket equip it. Get the spirit trinket from brewmaster Get to 10-11k with full raid buffs. Its easily doable with entry level gear. The second thing is stay in inner will and use PWS as close to rapture CD as possible and cascade as much as you can. Stack a lot of mastery. Don't touch crit blizzard ruined it completely for disc.
Reforge all haste to mastery and pick mastery if you can for enchants. Your target should be 13k spirit fully buffed with at least 50% mastery. After that you can continue to stack spirit and get haste or you can stop stacking spirit and just go for int and mastery.
Now what you are doing wrong in this fight is you are not using spirit shell correctly. Use it on CD chained with PI preferably and spread the buff out on 4 groups. Time it so that you catch jade shards. or overloads from non petrifying bosses if you ever get any. Outisde spirit shell keep stacking evangelism with penance and holy fire only then use archangel to fire cascades/PoHs on people taking damage. Make sure you use penance and holy fire on the bosses that are tanked together. You should be using AA at least 1.3 times per minute.
Yes the disc playstyle is really clunky and difficult but you can get close.
Thanks for the response, I'll have to work on that balancing act then!
Would you say that Cascade is the best end-talent to go for right now? It seems as though Halo does more healing, but costs significantly more mana.
My biggest concern with Halo right now is the position requirement. To get the most out of the heal you have to be 25yards away from the target. Well what happens when you have multiple targets? Who do you position yourself against? And you have to think of all that while dancing and juggling like a boss! Cascade just seems to be the easier choice for me at the moment.
I was having significant problems as disc on guards, so I went holy(Serenity Chakra) and found it much easier. From what I've seen with disc during my raids, is that we really don't have a solid spot heal. Heal/Greater Heal would be nice if they didn't take 2.1 seconds to cast. So you can fix that by combo'ing that out with borrowed time, but then you are spamming a 18k mana spell to try and keep your haste up. It just seemed like a crap shoot for such a spikey fight.
One thing that I've been doing in order to help with my mana is use Power Infusion as a mana saver rather then a oh crap button. Another would be to swap out FDCL for Mind Bender. Everything that I've been reading has been putting the mana returns (or mana saves in FDCL) from Mind Bender on top in that tier. PW:Give Me Mana Back can be effective when you have lull times during a fight... which you really don't have!
PW:S is really expensive right now, your usage doesn't seem out of the ordinary. My point was that we don't have an inexpensive spot heal, and some people would suggest blending PW:s with Heal/Grt Heal in order to get some use out of the haste but we all know your mana would melt away.
I get the feeling you are pretty set on achieving max healing while in disc, but I would really look into holy and a fantastic alternative for some fights. If you look at WoL, Holy is parsing much higher then disc on guards and feng right now. I personally thing that we (disc) can top Feng really easy but I wouldn't lock yourself to a spec when you have so many options at your disposal.
You are wrong PWS is cheap. Stay in inner will and spam it. If you get it fully absorbed its 15k mana for 80k. Its more mana efficient than gheal. With 11k spirit you gain 1k mana from it. If you don't use it you will suffer. Disc can produce high numbers on guard, but the playstyle that disc has is very difficult and the majority of players will fail consistently. I know exactly what to do and still fail from time to time.
FDCL is not good for disc, because you don't have any synergy between flash heal and your other spells. Mindbender for fights with a low to moderate mana drain, solace with fights with extremely high mana drain. Nothing else is viable.
Power infusion is the only viable selection in the tier for disc.
Holy is better than disc for most fights, on output, but I think that on that one they are nearly equal.
Gem all spirit gems. Get rid of your pure int gems. Spirit is our regen stat now, if you want, you can go int/spirit in the red socket, but if the bonus is ehh, I would still gem pure spirit. Try to farm the 5mans to get the spirit cloth gear, instead of having to reforge into spirit. Get rid of the egg and pick up a regen trinket (Price of Progress, Vial or barrel).
For stone guard, the dmg is very unpredictable and it is constant. holy shines here.