Before the pull
If you have an Onyxia Scale Cloak, put it on to prevent dying from Shadowflame. You shouldn’t take any of them, but better safe than sorry.
Depending on which strategy your guild is using, your healers may need to be in line of sight of the boss in order to heal the tank. If that’s the case, they’ll take debuffs and damage. This means they’ll have to hide at some point, so the debuff wears off.
In that case, it would be a good idea that healers don’t all hide at the same time, so the tanks keep getting heals. To prevent that from happening, you can divide healers in two groups. While one of this group will be healing tanks, the other will be hiding. They can rotate each 6 debuff.
The fight
There will be a lot of raid damages, but -contrary to Vael- you won’t have infinite mana. Use aoe heal as often as you can (downranked). When your party don’t need healing, follow the general gameplay advices already in that guild. It’s a rather long fight, so you won’t be able to only use flash heal – even though it’s tempting with those kinds of damages.
While you’re hiding, keep healing the raid members that are in your line of sight. When you’re out, focus on keeping the tanks up.