Here is another view of this setup from the mini map.

Ranged should be spread out in that huge are marked green, but still grouped enough so that their healer has range.

Melee is going to start at M-1 and then jump across the lava (marked by white arrow) the second the MT agros Rag. Melee should then be spread all around Rag but at least 15 yards away from MT1 and MT2 at all times. Don't cluster up on top of each other. Spread out all the way down the hook.

Melee healers can try to stand across the lava (marked M-H), but they will likely have to jump across (or get knocked across) to stay in range of their group. Plus, Prayer is going to work a lot better if they jump across.

The MT positioning is very important. They should never be within 15 yards of each other. One of the two should always be within melee range of Rag. Eventually MT1 is going to be knocked back and MT2 will have to immediately run in and get agro. After MT2 has agro, he moves to MT1's position and MT1 moves to where MT2 was. They rotate.

The MT's healers have it pretty easy. Put your back against that wall and spam heals nonstop on the tank. They can and will take 4000+ dmg instantly. The MT needs constant spamming of flash heals with druids and paladins padding or they are going to die.

You are going to be knocked out of position by a random explosion at least once. Just immediately run back to where you were. You are often thrown into the lava so make sure you are capable of getting out. Go practice.

Before you even spawn Rag get into the position described. Get everyone exactly where they should be while fighting. Look for people that could be spread out more, people who aren't in range of their healer or aura/totem, and other easily corrected mistakes. Practice the position and be 100% sure your entire raid knows exactly where they need to be.

When practicing, make your melee players jump from M-1 into M-2. If you jump in the lava correctly, you won't take any lava damage what so ever.

This entire 3 minutes is all about positioning. Know it, practice it, and execute it correctly.

Plan:

Buff. Buff like you've never buffed before. Every single melee should have bok/might/salv/light. Every single caster should have bok/salv/bow. Everyone should have MoTW/AI/PW:F. A fully, completely, totally buffed raid is a happy raid. We like to buff up in the west (like in the positioning screen shot above). Once you get blessings, move into position.

If you're going all out, remember to drink your FR potions 2 minutes before engaging since they are on the global potion timer cool down.

Rag is up. MT1, MT2, and their healers are all hugging the south wall. All the melee players are in M-1. All the ranged are spread out in the west near their healers.

0:00 - MT1 runs into position and gets agro. CT_RaidAssist should start giving you information the second Rag turns around to attack.

0:01 - MT2 and all the melee players run/jump into position and start attacking. They are nicely spread out, in range of their healers, and doing excellent DPS without pulling agro from the MT, all within a few seconds.

0:23: The first Wrath of Ragnaros is about to happen. Everyone except MT1 moves out of melee range. Ranged keeps attacking.

0:25: Wrath has hit MT1.

One of two things has now happened. Either MT1 did not resist wrath and has been knocked back, or he did resist it and is still happily tanking away.

If MT1 has been knocked back, here's what needs to happen: MT2 (who has backed off like all the other melee) needs to immediately run in, get agro, and move to MT1's position. He will spend a moment building threat and call when melee can resume attacking. He should be #2 on the hate list because he was attacking for that first 23 seconds like everyone else.

Once MT2 calls the attack, all melee resume attacking. Ranged never really need to back off, but they should not be doing any big damage for that 5-10 seconds around a wrath. Ranged really doesn't want to pull agro.

While MT2 is getting into his new position, the knocked back MT1 should run back and go to MT2's old position. They just rotate after a knock back.

Fortunately, most of the time a MT with 350+ FR will resist the knock back.

If the MT does not get knocked back, melee can resume attacking in about 2 seconds. Wrath definitely reduces everyone's threat on Rag's hate list, but it does not completely wipe. Just give the MT a couple seconds before resuming good DPS after a resisted wrath.

That's all there is to it folks. Cycle through this pattern over and over until 2:50 into the fight.

Wrath->check for knock back->move MT2 in if needed->wait a few seconds for MT to call the attack->attack good DPS for around 20 seconds->move out of melee range->repeat!

Wrath is on a 25 second cool down but he will not use it exactly every 25 seconds. More like every 25-30 seconds. Wrath will hit several targets within melee range. You only want 1 person in melee range (your current MT) when wrath happens.

Your MTs have to be on the ball, ready to transfer the job after one of them gets knocked back. If the MT1 is knocked back and MT2 doesn't get into melee range immediately, Rag is going to start Magma Blast oneshotting random people since no one is in melee range.

MT healers need to be ready to switch targets very quickly. The guy does a ton of damage and if one of your MTs dies, you're wiping.

Your melee players have to be ready to move. Don't wait that 2 seconds for that 20 energy tick to do your last ability. Get the hell out of melee range the second the script tells you to. You have to stay alive.

Mind your DPS but do a lot of it as safely as you can. If you pull agro, you will die. Rag's target requires several healers constantly spamming heals to keep them alive. You will not live if you pull agro, period.

At 2:50 (10 seconds until Sons) ranged collapses into sons position. Hunters lay frost traps pretty much anywhere along the lava if you can. At 2:55, melee collapses. The MT and his healers do not collapse until Rag actually goes underground and the Sons spawn since someone has to be in melee range at all times.

Ramblings:

There should be very few deaths during this phase because nobody but the MTs have been eating wrath, and healers have been keeping HP topped off from AE fire damage. If more than 3 people are dying in the first 3 minutes, there are problems with positioning or healing (or simply a lack of FR).

There is an unfortunate amount of luck involved with this "phase" of Rag. There will be runs when the MT is getting knocked back all the damn time and your DPS sucks because you have too much downtime switching MTs. Or maybe your MT healers will eat like 5 random explosions in a row and waste all their mana healing things that aren't the MT. Or maybe a random explosion hits all your melee players at 2:58 as they are clustered up collapsing for the sons. Sometimes the lava splashes are just out of control and all your melee healers get cooked with the melee players dying shortly after to AE damage that adds up.

We've seen it all happen. Our typical runs will see the MT knocked back 1-2 times and not be a big deal. We'll lose a few precious seconds of DPS when we have to switch MTs, but if it's only a couple times, it's not bad.

A few people (usually melee) are going to get owned with some random and disgusting combo like: explosion->lava splash mid-air->land in lava->take a lavadmg tick->dead. It happens but it shouldn't be happening to more than 2-3 people, sometimes not at all.

Biggest problem will always be getting melee players on the ball and out of melee range the second they are told to get out. It's simple, if you get Wrath and you are not the MT, you have made a mistake and are going to die.

Typically he goes under at around 30% if we are going to kill him that run.
http://www.marsgamer.com/freeguides/complete_guide_to_ragnaros_molten_core.html

The MT positioning is very important. They should never be within 15 yards of each other. One of the two should always be within melee range of Rag. Eventually MT1 is going to be knocked back and MT2 will have to immediately run in and get agro. After MT2 has agro, he moves to MT1's position and MT1 moves to where MT2 was. They rotate.