When I heal this on my paladin on 10 man, i take the tank group and my fellow healer takes the other group.
We restrain ourselves to crossheal, but we will do that, when a person with 1 stack drops dangerously low.
To keep the tank alive during purple, a paladin has quite some tools.
1. Beacon the tank, beacon heals don't add stacks
Always be healing something, be it one of the other group members or even a pet, to keep healing the tank constantly
2. HoSac the tank when he gets like 3 stacks of the deep corrption, at which time you have to be a bit more carefull with your direct heals on the tank
3. Use LoH and maybe even GotAK, preferably on purple-red-black, as this is the only purple phase, where you have 2 damage sources, while having the healing debuff (the other purple combinations are purple-blue-damage source, which are more forgiving)
4. Use holy shock to get holy power stacks, but NEVER on the tank , as the low heal does not justify another deep corruption stack. Use it on raiders with low stacks or a pet instead.
5. WoG doesn't add stacks, so keep spamming that on the tank (LoD during the phases where you don't have purple)
6. Stand in front of the boss, so maw of the dragonlord cleansing flames hits the whole raid (on top of him during red/black, a bit behind him on green)
7. Keep your divine plea for when you get the first manavoid, after that use it on cooldown when you are running to kill te next slime.
8. Do not be afraid to heal the tank. throw him a divine light everytime he dropped more then 30K health. When you let him drop to less then 50%, then you better have LoH ready
The other mechanics involve tanks popping their own cooldowns and using raid cooldowns that are available at times where you don't have LoH or HoSac to help out
