We had some trouble on this fight too in the beginning, people claimed it was due to low dps and every reason there are.
We found out that a few things that will probably help you out a lot is to use 3 healers as long as you can finish before the 3rd pool adds spawn. And do not use BL instantly in the last phase, use it when healing becomes a problem (usually after the first tentacle spawns) this will buy you about 20 seconds more. Have the shadow priests use divine hymn in the beginning of P2 to make sure that everyone is topped properly which might even allow BL to be used later.
Ideally you should end the fight with a few seconds of BL still left, not gone since 20 seconds since the fight scales healingwise and you need more CDs in the end to kill him and BL is the biggest healing CD there is
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Hope this helps and good luck.
Oooh, switching one of the spriests to a Disc priest (atonement) as the third healer is awesome, with the proper specc he can easily pull about 9-10k dps and still heal very very well in the last phase.