Before the pull
During this fight, someone will be controlling the Orb to MC the boss. While doing so, he will take damage that may interrupt his cast. To avoid that, a shield must be up on him all the time. You can assign one of the priests to this task before the pull. Although every priest should watch it just to be sure.
The Phase One
The raid will only take few damages during this phase. It’s not healing intensive and you should mainly be saving your mana for the second phase, as well as watching if the orb guy is always shielded.
Nonetheless, it can happen that one of the mob or even the boss (if the MC breaks and isn’t taken back fast enough) attacks a healer or cloth dps. In that case, you have to be ready to save this person. This means using powerfull and fast spells such as shield or flash heal. It costs a lot of mana but you don’t have the choice in the case someone is about to die (and you can die from only one or two hits from razorgore if he’s lost)
The Phase Two
This is the phase you saved your mana for. You should have no less than 80% mana.There will be a lot of raid damages and a lot of people may los you in the process of trying to los the boss. It’s a fast phase with a lot of raid damage : try to be as effective as possible HPS wise. But still keep an eye on your mana pool and go back to efficient heals mana wise if you see it dropping too quick and that you don’t have mana pot available.
The trap of that phase is that you may forget to heal the tank because of the raid damages. So keep an eye on him! The other difficulty is, as said above, the fact that some people will be out of your line of sight, which can be quite annoying.