This boss is extremely challenging... even more so than Koralon. However, there really isn't much strategy for this boss. It's all about having enough health/mana and primary stats to survive the damage. Boss doesn't hit for quite that much, but his four adds each deal 3k+ per hit, which adds up to be an extremely healing intensive encounter. Also, killing the adds will immediately result a new add taking its place, hence there will always be 4 adds alive at any given time. One way to slightly soften the incoming damage is to pick up Inspiration talent in the holy tree, however doing so will result in less mana regen due to lack of Archangel, hence SW:D is casted more often and Dispersion is glyphed.
The fight is actually all about timing Defensive CDs to maximize their usage throughout the fight. This means using defensives early on in the fight so they can be used again later on, despite the fact that defensives aren't really needed at the start of the fight. Think of it as using Heroism/Bloodlust at the start of a Madness of Deathwing encounter, it really isn't needed, but by doing so it can be used for a second time near the end of the encounter. Dispersion is the prime example here, I used it early on just so I can squeeze in 3 of them throughout the fight and that allowed me to gain an extra 36% mana back.
With that said, the boss has two main abilities to watch out for. The first is a Lightning Nova ability. When he starts casting this, move away from him to avoid the damage. It is also advised to use some instant cast heals while moving. The second main ability is used after his first one, he will empower one of the 4 adds, healing it to full health. The add will now gain a stacking buff which increases its damage done. When the buff reaches 10 stacks, the add explodes, dealing 30k damage. There are two options to deal with this mechanic. The first option is to burst it down. It takes approximately 4 GCDs to burst it down. This means 4 GCDs without healing. For a healing intensive encounter of this magnitude, 4 GCDs without healing can take out more than half of your health bar, so definitely don't opt to burst it down if you aren't topped up. The second option is to just heal through it. This is a mana exhausting option, but has a higher chance of surviving through the mechanic.
My method of dealing with the empowered adds:
1st Empowered Add: Use shadowfiend+fade, and then burst it down
2nd Empowered Add: Heal through until 7 or 8 stacks, then Dispersion the exploding damage
3rd Empowered Add: Either burst or heal through, depending on the situation. Use whatever CDs available.
4th Empowered Add: Dispersion and Desperate Prayer (I had no choice but to use Dispersion and Desperate Prayer early in the video since I had literally no mana).