This boss is divided into 2 phases, which phase 1 further divided into a ground phase and an air phase. Phase 2 begins when boss reaches 35%.The boss also has a 10 minute enrage timer, which is rather tight for priests. Hence it is recommended to reach phase 2 with as less air phases as possible during phase 1.
My first unsuccessful attempts all hit the enrage timer due to having an extra air phase, despite the transition to phase 2 being extremely smooth. I finally managed to kill him after pushing for 1 less air phase, however my transition to phase 2 was just about as worst as it can get (CDs not up, health not topped off).
Sindragosa herself does not melee as hard as the other ICC bosses, at least not enough to trigger the Masochism talent, however there are other damage sources such as raid wide aura, blistering cold, unchained magic and frost breath. This means that mana should not be used as mindlessly as on fights such as Marrowgar.
During phase 1, Sindragosa will use Unchained Magic, Frost Breath and Blistering Cold. Unchained Magic is a 30 second debuff that causes all spell cast to generate a second debuff. This second debuff only lasts 5 seconds and deals a small amount of damage when it expires. However, if another spell is cast within this 5 seconds, the second debuff will stack, increasing its damage done upon expiration. As a priest, there is no way to deal with this debuff (can't stop casting and let the boss melee you to death just because you have the debuff), and hence its damage must be healed through. Frost Breath is another attack that the boss uses.The damage isn't that high, but it leaves a nasty stackable debuff that lasts for 90 seconds. This debuff reduces the movement speed and melee haste. The melee haste reduction doesn't apply, but the movement speed reduction is quite annoying. At 6 stacks it becomes almost impossible to move. Blistering Cold is the final noticeable ability she uses during the ground phase. It is basically an AOE attack that deals around 30k damage to anything within 25 yards of the boss. It has 5 sec cast time so it should be rather easy to move out, but if there is more than 4 stacks of Frost Breath debuff, then don't bother.
About 50 seconds into phase 1 and every 110 seconds afterwards the boss will fly into the air and enter air phase. During this phase she normally selects random raid members and iceblock them. She then casts frostbolts on the ground that cause heavy raidwide damage unless players stand out of line of sight behind the iceblocks. However when soloing, she does not iceblock the player and hence all the frostbolt damage must be taken.
Phase 2 begins at 35% health and for some weird reason, she no longer casts frost breath. She also seems to often try and cast Ice Tomb but since there are no other players in the raid, this is also unsuccessful. However her other new ability Mystic Buffet, causes a 10 second stackable debuff which increases magic damage taken by 20% per stack. Normally the stacks can be reset by standing out of LOS behind an ice tomb. But for soloing, this debuff cannot be reset. This effectively puts a soft enrage onto the fight in addition to the hard enrage.