210156
The key healing on this fight comes in two forms: first you need to cover the damage from breaking the 422261s. Each 422261 will heavily damage the group and then apply a debuff that makes other breaks way more deadly for a couple of seconds and so the way to handle this is that you need to quickly top the group between breaks so they can keep killing them as soon as the debuff allows. If two 422261s break at the same time and hit the group for a huge amount, this is a dps missplay and not a healer check.
The 2nd part comes from 423200. Approximately 40 seconds into the fight and then repeating every minute the boss will absorb all remaining 422261 and shield himself, followed by doing a lot of damage to the group via 423324. This will continue until you break the shield, and will keep increasing by 10% every tick.
To help you deal with both of these mechanics, 423557 can give you a 50% healing increase for 30 seconds when you grab it, making it way easier to deal with the incoming damage but still requiring you to plan your cooldowns in a smart way around the constant spikes of heavy damage the group will take.
213119
The main damage of the fight will come from 427329, and the main issue will come from the fact that people are constantly spreading across the room either to touch portals and drop their debuffs or to drop puddles on the ground. As you can imagine this is very annoying for Preservation but we have a perfect way to deal with it: lb.
Once applied both echoes and lb work up to 100yds and so you can freely echo your group when everyone is stacked in the middle of the room hitting the boss and then when the time to run around comes you can consume them with ve and keep healing everyone by sending spells into the lb, either ec if its a big emergency or sb for normal healing.