For 10-mans, this is a 2-healer, 1-tank fight, having 3 healers is redundant (Either you're OOM or you're burning the majority of your mana anyways during a heavy aoe phase which will be redistributed anyways, and while the latter might be slightly easier with 3-healers you're still throwing cooldowns left and right so it's not as bad as you think) and you won't have the DPS to beat it otherwise.

In terms of debuff from the purple ooze for 10-mans, the easiest technique with the least amount of communication is basically what's suggested here, assign one healer per group. As a paladin I'm in the tank's group and the best result, I found, was divine lighting the tank and immediately following up with a WoG (from ToR) on the tank or another party member depending on their health, even healing a member of the other group (Since Word of Glory doesn't proc corruption stacks so you have no issue with causing your other healer's group any undue stress), and then holy shock any non-tank raid member and repeat with WoG.

What this guide doesn't tell you is that the debuff resets mid-phase, so keep an eye on the debuff's duration when judging how high you want to get those stacks up. Another tip is realizing that the tank doesn't get stacks from beacon heals, so you can holy light spam a pet (which doesn't get the debuff) if things get tight.

Another general tip is to try and stay close to the mana void when it bursts, the thing CAN sucker you out of all your mana.