We have been trying this boss yesterday on heroic and we have some serious issues. The main issue is probably the fact that we are using a DK tank.
We are rolling every single cooldown possible on our dk tank mainly, hand of sacrifice, hand of protection + ps; but despite that once the cooldowns are off, it becomes very hard for our healers to keep the voodoo doll targets up.Note: we are using 2 healers for this fight, they are pretty good healers and random burst damage is the only reason of deaths.
2: Your DPSers are shit and don't clear all the shadow minions in the spirit realm / You skip a totem at one point casuing the shadow minions to become too many.
3: Your tank is terribly geared / can't play.
4: When your healer goes into the spirit realm, he stays too long. He should heal up the people inside so their button appears and then exit asap to reduce the amount of time where it is only 1 healer outside healing tank + voodoo dolls.
We did our first heroic gara'jal with a rshaman and resto druid healing and a DK + warrior tanking quite easily. You have a paladin + disc priest healing, the best possible 2 man healing combination you can have. So you're either undergeared or it is one of the reasons I mentioned.
We use a resto druid+shaman and a smiter disc on this fight, and can comfortably beat the enrage. I advise you do the same, since there are times when the disc has to go down, because the druid has the debuff from earlier, and the shaman has voodoo doll. And more importantly this way you get some extra cds, and two healing (or even solo heal+attonement) the normal world is much simpler, because you can just spot heal the voodoo dolls, and the attonement heal will take care of the others.
As far as the dk goes, we actually use one with a warrior as the other tank, and not only noone dies, but on the last 3 kills, the dk took the boss during frenzy, and still managed to keep everyone alive.
As a druid, I use the 4 piece pvp set on that boss (since mana is a non issue), which reduces the cd on the 20% dmg reduction cd by 90 sec, so we can use that every 30 sec. Even if you don't have a druid, it shouldn't be that much of an issue. I've seen ppl drop in 1 global even when the warrior tanked the boss. He probably screwed up, but it happens to nondk tanks too.
tl;dr, use 3 healers, and imo your issue is probably the dk player, not the class itself. Like I said, even a dk can tank the last phase without any kind of problem.
At this point, unless you're terribly undergeared as a raid, there's no reason to fail at this fight. Gear has made everything that was difficult about this fight a complete non-issue.
The Voodoo players still get hit quite hard, but as long as you guys call for cooldowns appropriately, and your players with voodoo use their own appropriately, it ends up being whatever. It may take them a few pulls to get used to it, but I can't imagine it being a problem with gear.
We went in there a few weeks ago with average raid iLvl on about 485 and killed it in 5 attempts. 2 healers, Disc + Monk. As the Disc I just PW:S the 3 voodoo targets every time it's up and get big spirit shells on them, they take 0 damage
Since we 2 heal sometimes a healer can't go down. If this is the case we send our Moonkin down solo, he can easily top himself with NS + HT then healthstone. Sometimes he can't go down either we just skip a totem and send down our Moonkin and Warlock to kill a million adds.
But really the fight is so quick...we burn him to like 60% before we take the first totem, and you burn the boss at 20%. You barely even have to take 4-5 totems.