Phase one of this fight is basically a balancing act, you have to make sure you have enough dps on the boss full time to burn her mana shield and also enough dps on the add to take them down before the next wave comes out. You'll want to have any dps classes that really have to rev up to get their dps up there on the boss full time, because the ones who aren't full time on the boss will be switching targets a lot. An example of a class that would take a while to rev up their damage is moonkin.

In 10-Man mode the adds will only come out one side at a time and they will alternate between left and right side of the room always starting on the left side. The Left side of the room will always spawn 2 Cult Fanatic's (Melee Adds) and 1 Cult Adherent (Caster Adds). The Right side will always spawn 2 Cult Adherent's and 1 Cult Fanatic. On 25-Man mode the adds will come out of both sides each wave and 1 add will always come out of the back, the add that comes out of the back is random.

Lady Deathwhisper also uses an ability during this phase called Curse of Torpor which will lock out the next ability you use for 15 seconds, that is why de-cursing quickly is very important. In my video above I didn't get mine de-cursed fast enough and I locked myself out of Wild Growth, if you look closely you can see it on cool down.

Stay out of the runes of death, they are big they are green they are hard to miss, if your paying attention you shouldn't have a problem with them.

On 25-Man mode you will want one tank for each side of the room (Left, Right, & Back) Or you can have a hunter use misdirection on the back add to one of the tanks. You will want one healer on each tank and the rest raid healing. If you are raid heals I suggest you pay attention to people calling out when adds get empowered, deformed, or reanimate and head to the side which they are on to assist in healing.

She has 1 more ability on the 25-Man mode that she does not have on the 10-Man mode which is called Dominate Mind which will take control of one of your raid member and he will begin attacking/healing for lady deathwhisper's team. It is very possible to kill them while they are under the effect of this charm and very impossible to heal them. However this charm is not permanent so you will want to cc them and make sure not to kill them, they will be fighting for you again soon.

You will want to stop dps on the boss when her mana shield is around 10%-5% and get all the adds down then if you have time to finish off her shield before the next wave of adds, pop heroism / bloodlust and take out the shield, thus triggering phase 2.

Phase 2 is pretty much just dpsing her down, the dps and tanks will have to interrupt some frost bolts and stuff but that is not really your concern as a healer. What you need to know about this phase is that it is a lot of raid healing. She will be casting out frost bolt volley whenever they get past an interrupt which will do a lot of raid damage. You still have to worry about staying out of runes of death as well as there will be ghosts spawning all over in your raid and running after random raid members, if they reach that member they will explode doing damage to them and everyone around them. If you run away from the ghost they will disappear and do no damage, however they are hard to see so this isn't easy.

This fight was not near as hard as the Marrowgar encounter and not quite as different as they made the Marrowgar encounter either. I'll start in phase one.

This phase went exactly the same as in the regular mode, you burn down the mana shield while you take care of the adds. There is something that sets this phase apart from the regular mode however. The adds have considerably more health then they do in regular mode so you really have to get them down quickly if you want to get the mana shield down in Time.

Also she now uses Dominate Mind in 10 player mode, which she used to use in 25 man regular mode but not in 10 man regular mode, so if you have never done the encounter on 25 man mode this will be new to you. Dominate mind will mind control one of your raid members and this raid member can be killed easily however the mind control is not permanent and you will want to just CC the raid member until they come back to their senses.

In phase two you are going to be faced with a few different changes. In regular mode you basically have a dps burn on the boss and its an easy kill, not so much in the heroic mode. During this phase in heroic mode, instead of the adds not spawning any more they keep spawning but instead of 3 spawning at a time from either side of the room there is only one at a time coming from the back. Also you remember the ghosts that spawn and explode on contact? ya well in heroic they now have a chance to 1 shot you, so run like hell and they will de spawn. She will also continue to use Dominate Mind during this phase.