Before Pull
Pop a shadow protection potion.
The fight
If the decurses are done correctly, it’s a rather easy fight as healer and you won’t have much to do.
This means that you’ll have more than enough mana to dispel melees. The magic debuff they’ll get throughout the fight will decrease their attack speed, so dispelling them will increase the raid dps.
If a curse explodes, you’ll have a lot of raid heal to do, otherwhise it will mainly be tank healing.
When the boss disappears, there will be some shadow damage on raid to heal up. When the boss comes back, you have to get out of middle not to take agro from renews and stuff.
After blink, the boss agro resets, so keep an eye on the boss’s target. If it’s a caster, shield him and start precasting on him.