Before Pull
Pop frost prot pot and get HP consumables.
If your healing gear isn’t that good you can use some frost res gear. But usually (same as viscidus), it’s more mana efficient to keep real gear and put a little more mana on yourself. The only risk by doing so would be to die from blizzard.
Phase one: sapphiron on ground
The raid is spread in 2 groups: left and right side of sapph. So it’s probably a good idea to have a raidframes allowing working range check.
Basicly everyone is getting frost damage, so the most efficient way to heal this is to spread renews (without overwriting others) and use PoH.
If there is a lot of priest on your side, that they also want to spread renews, and that they are better geared than you: let them do most of the renews and focus on tanks. Otherwise, your side will be full of renews on everyone and the better geared priest won’t be able to use his gcds (less powerfull renews rolling on raid).
In any case (that you’re spreading renews or not), always keep the tank up, precast on him from time to time.
Basicly, just try to look and know what other healers are doing around you. Because everyone is taking massive damage on this fight. So if everyone focus on healing tank, it won’t work. And if everyone is focusing on raid it won’t work neither. On this fight, you can very much top heal by spreading renews, but it’s kinda useless if the tank dies. So yeah, try to analyze what’s going on in other healer mind, and do the left over job.
Other than this, watch closely people affected by curse / blizzard and throw real heals on them (not only renew)
Watch blizzard for your own safety as well. It has a huge range, and moves progressively toward left or right of it. Have max camera distance enabled and try to move preemptively. If it pops right on you, move toward the right direction, not toward its center.
Phase two: sapphiron flying
There will be Ice Block target, behind which you have to hide before sapph does its aoe thingy.
The ice block are kinda bugged on feenix, so try to be spread out during this phase and do not move until you have to go hiddding (which is when all the icebloc kare visible).