Group Setup:

Assume I say Shaman everywhere I say Paladin if you are Horde. But no Horde will ever read this guide since 4 Shaman chain casting frost shock can drop Rag no problem, right.

Melee groups and ranged groups is the general idea. MTs, MT healers, and melee groups should get priority on Paladins because they will need FR aura more than ranged. Always use FR aura/totems.

Groups 1 and 2 are MT groups and should consist of: MT1/MT2/Paladin/Druid/Priest and Paladin/Priest/Rangedx3. Healing on MT should be 2 Priests, 2 Paladins, and 1 Druid. The 3 random ranged filling up group 2 need special attention from the designated ranged healers since their in group healers will be entirely focused on the MTs.

Then around 3 groups of melee setup like so: Warrior/Rogue/Rogue/Paladin/Priest for aura and battle shout. Give melee groups Priests because they will take AE damage from Lava Splashes and Prayer of Healing comes in very handy.

And the last 3 groups or so are Mages and Hunters, likely with no Paladin and a Druid healing them. They definitely take damage from random explosions that needs to be healed, but they shouldn't be close enough to the lava to take Lava Splash damage.

You will have a warrior MA for the sons. Set that up ahead of time and make sure he has good healers.

Here's a example raid group screen shot.

Positioning:

This is going to look really confusing at first but it's quite simple.

http://www.marsgamer.com/freeguides/complete_guide_to_ragnaros_molten_core.html

And the last 3 groups or so are Mages and Hunters, likely with no Paladin and a Druid healing them. They definitely take damage from random explosions that needs to be healed, but they shouldn't be close enough to the lava to take Lava Splash damage.