This is it -- the last priest healing boss guide of the expansion! (After this, it's pandas or bust, baby!) Are you excited? You should be, because the last boss fight of Cataclysm is
Priests of either spec should know that cooldown management is the key to succeeding in this encounter as a priest. The encounter as a whole is not particularly difficult, but using your regen cooldowns at the right time will pave the way for you to hold onto just enough mana from start to finish. I recommend using your first Shadowfiend after the Mutated Corruption dies on the first platform, and your first Hymn of Hope just after jumping over to the second platform. Your second Shadowfiend should be used as soon as it becomes available, which will allow you to pair your third Shadowfiend and second Hymn of Hope together (just like we used to always do it in Wrath -- cast Shadowfiend, then immediately follow it up with Hymn of Hope). From there, use everything on cooldown, and you should be sitting pretty for mana the whole fight.