Lady Deathwhisper's chamber has a few trash packs that should be pulled via line of site if possible. For us, they always bug out and run to the right side of her doorway, even if the raid is huddled around the left side, so we find it better to always hide on the right (from the perspective of the hallway). Focus fire down the casters first, but really just target your tanks' targets...everything hits like a truck, so the important thing is just not to pull aggro off the tanks. No need for more than two healers though. As I recall, someone should be prepared to de-curse as well.
Make sure you pull the big adds (one of the left, one on the right) by themselves. They each do significant ticking raid damage, and each will cast a spell that puts a bomb on one player at a time. That player needs to quickly run out of the raid and get quite a ways away (I suggest going back up the stairs, out of the room) before they explode. Healers should drop a HoT on them before they get out of range.
Raid Comp: 2 Tanks, 2 Healers, 6 DPS
Lucky Numbers: Don't stand in death and decay!
Phase 1: The Shield and the Adds
Then comes Deathwhisper phase 1. She will put up a magical shield, and all damage to her drains her mana instead of health. She'll also be casting shadowbolts and death and decay (don't stand in it!) through the encounter. Still, two healers should be enough. Be careful though, she will sometimes put Curse of Torpor on someone. This puts a cooldown on each ability you use while cursed, a full 15 second cooldown I believe. Make sure it gets decursed before you do anything important, especially if you're a healer. Whoever is decursing, make sure to decurse yourself first!
All dps should be on the shield until adds spawn. Three will spawn on the left (two melee with a caster between them) and all dps should switch to them as soon as the tanks have aggro. Take the caster down first. Sometimes Deathwhisper will empower the adds, making them do AoE, so ranged should stay back and healers should stay alert. Melee should run out if they see themselves dropping precipitously. Some adds will get brought back to life if they aren't all killed fast enough. They may be brought back as giant skeletons, which will be immune to either magic or physical damage, so figure out which ones you can hurt and work accordingly. They may be brought back as limping fleshy thingies. Those must be kited by the tanks. They hit like trucks. Big fleshy trucks.
Once the adds are down, dps should switch back to Deathwhisper until more adds spawn. Eventually, the shield will come down. Try to time this for just after you've finished killing a wave of adds.
Phase 2: The Burn and the Ghosts
Once the shield is down, tanks need to taunt Deathwhisper quickly. She will cast frostbolt volley, with is raidwide but not too bad, and death and decay. She will also cast a massive single target frostbolt on the tanks. This must be interrupted. Every time. Or you'll die. Horribly. It does up to 50k damage. Tanks need to trade her back and forth because she stacks a debuff on them that reduces their threat generation by 20% per stack. I'm not sure how many stacks we allow before a switch, but you should see how long it lasts and how often she applies it and trade at the minimum required to let it fall off before taking her back.
The only other tricky thing is that like blue translucent ghosts spawn during this phase. They are very hard to see, and they do a lot of damage if they hit you. They need to be kited by whoever they target for a few seconds before they explode. Else they stand a decent shot of one-shotting a non-tank, or at least making the healers miserable. Again, they are really hard to see, so be on the lookout.
After she's dead (she leaves a boring corpse...just a little sphere of darkness), the elevator in her chamber will start up, and you're off to the gunship battle...after you fall off the first couple times.