Our guild run this with 3 healers (Holy paladin, Disc Priest & Resto Druid) andd 1 tank (Feral), we always arrange groups so that healers have assigned targets to heal during purple phases, circle cds on the tank (PS and Sacrifice), also haivng a holy paladin makes the purple oozes a joke. Word of Glory and Beacon doesnt stack the debuff, just have him heal a pet and the other 2 healers take care of the groups. Using 3 healers makes the *hard* ooze combos like yellow, red and black easily manageable. Set up cd rotations and then just heal like mad men.
About tanks I have heard that DKs make the fight alot easier however, we dont have much trouble using a Feral druid and he tanks all the single-tank fights.
Also, we leave the melee on the boss and have our arms warr charge the ooze IF the range would require help (We run between 2-3 melee but on the first kill we had 2).
I hope you kill him and have your healers look up which of their abilities that procc the purple debuff, as it would seem you explode more than needed.