Before Pull
Pop a Nature protection potion.
Assign someone to cure diseases, to renew diseases target and to shield diseases target
The fight
There will be diseases poping on random raid member. Said raid member will have to move out of the raid and gets cured once he’s far enough. When he is cured, he will take damages - that should be healed up/absorbed if he got pre-shield / pre-renew before going away.
Otherwise, keep an eye on melees and on the tank that may take damage from the green stuff. You can pre-renew melees during the chill period of time.
Watch what other healers are doing, if the raid heal is good, pre cast on tank. If the tank healing is good, spread renews during chill period and top melees with heals/HG when the damages happen.