Edit: Ofcourse, I misread what quest you were on, the advice below is sitll valid for the Celestial Challenge
Here is what I used in talents:
Disrupting Shout (interrupts)
Shockwave (stuns boss + adds)
Avatar (macro'd to Skull Banner & Recklessness for instant burst)
Second Wind purely for the higher potential healing
250 Mastery food (still had it on from LFR)
I had some issues with this fight as well, particular because we're virtually the only tanks with no self-healing.
As a result, you need to rethink your entire strategy.
Deathwing himself doesn't hit that hard, up to a point where I was using Shield Barrier over Shield Block for the extra total reduction. Remember, no heals = damage smoothing doesn't matter. Not to mention that because Shield Barrier has a minimum value, it actually works extremely well on the weak hits Deathwing does.
The blood adds hit a bit harder, which I kept Shockwave for, followed by Thunderclap. That should bring them down enough to Revenge them down quickly. If the boss runs after Wrathion, Intervene, taunt and Heroic Leap back.
On the Shard intercept phase, simply stay close to Wrathion and step out 2y in the right direction to intercept. Use your Safeguard or Intervene here if you ran off too far and doubt you can intercept shards coming from the other side.
Don't expect to get a lot of Vengeance, so make sure you prioritise Shield Slam and Revenge for the Rage to put into Barriers. Without the AP from Vengeance, Battle Shout is not much of a boost, so go Commanding Shout.
Demo Shout as much as you can, use Demo Banner or Shield Wall once the SW stun runs out on the blood adds to reduce damage taken. Keep Last Stand for when you do drop low to boost Second Wind more. If you really have issues, don't be afraid to use a Flask or elixirs, ask friends and family to buff you before you start, especially a Mastery and stats buff could help you.