Most 5-man bosses (heroic and non) are cake for Restoration Tree Druids to heal through. They all have their quirks, but generally we keep our Lifebloom stacked and Swiftmend when appropriate and everything goes well. Perhaps we even chuckle a bit when we get that “increased cast time” debuff. “Oh noes, my instant casts are…still instant cast!” But there’s one boss who can be a real pain if you don’t know what you’re coming up against. That would be Dalliah the Doomsayer of the Arcatraz. This is a guide for those Resto Druids who are Treeform Healers and have never had to come up against this particular boss but are planning on soon/eventually (you need to do Arc for your Kara key and she drops a good off-hand and on heroic a nice healing gem).
Two of her abilities are not your concern. These are her Whirlwind (you should not be standing close enough to her to get hit by this) and her Heal (which is the responsibility of other group members to interrupt; you don’t have one and need to concentrate on healing anyway). The two of her abilities you need to watch for are Gift of the Doomsayer and Shadow Wave.
Gift of the Doomsayer is the reason Treeform Druids have such an issue with this fight. Periodically during the fight she will cast this debuff on the target highest on her aggro list. This should be the tank. When this debuff appears on the tank, all healing needs to stop. This is because this particular debuff (looks similar to Shadow Protection buff with a red box around it) gives the caster (Dalliah) a healing every time the affected (your tank) is healed. This means every potion used, every straight heal and every tick of an HoT will heal Dalliah. Any healing done with this debuff on needs to be emergency only, as it will only prolong the fight. Therefore, it is important to try something new.
Get out of Treeform.
Whenever the debuff is on the tank, do not heal. When the debuff is off, however, cast Regrowth + Swiftmend, then Healing Touch. If your tank is really low on life, Rejuvenation + Swiftmend for a quick jump can be substituted. Repeat Healing Touch if necessary, but be ready to cancel it if the debuff reappears. This is the best way in my experience to heal through Dalliah.
The other attack you have to watch for is only in heroics, and is not as situational as the Gift of the Doomsayer debuff. This is Dalliah’s attack “Shadow Wave.” There are two reasons you need to watch this: 1) it attacks a target for 2500 and gives a debuff lowering Shadow Resistance; 2) it targets the player furthest away from Dalliah. Do not let this be you. Whether you’re with a PUG or Guildies, explain to them this particular nuance, decide appropriately who will be in the back (either due to high Shadow resistance or high HP) and then make sure this player is kept topped-off. This player will not receive the Gift of the Doomsayer debuff, and therefore can be HoT’d to your heart’s content.
As long as your tank and melee immediately exit Whirlwind and someone interrupts her Heal, Dalliah should be an otherwise easy fight.
To add, Dalliah is immune to taunts. Therefore, if you're worried about your tank (most likely happens in a PUG, not with experienced guildies) bring something like a Hypnotist's Watch to lower your threat. Also, your tank, if preferred, could always just "click off" your HoTs, but it adds much more hassle than required than if you simply do not add any.