This is one of those fights that is really about repetition. Do each of these phases enough times and you WILL get it down. The only problem is in player consistency. Rag is very much a patterned fight. Which is dangerous. You can be lured into a false sense of security and then completely not notice that Engulfing Flames is spawning under your feet. Having all DPS alive is a big priority in the last phase, so even losing just one or two - even in 25man - will pretty much guarantees a wipe (except not really anymore, thanks nerfs).
Ragnaros can be broken down into five chunks: Phase 1, Transition A, Phase 2, Transition B, Phase 3.
Run in (not before the tank, though if you have nice healers you will live through one stack of Burning Blood. Not that I speak from experience...) and plant yourself as close to Rag as you can get without swimming in the lava. My fellow melee and I stack together, which makes the placement of Sulfuras Smash predictable thus much easier for the ranged to dodge the resulting shock waves. Because you are hugging the front edge of the platform, you will always be in front of Sulfuras Smash and will never have to worry about getting hit by it.
Fire traps are put down, but let those ranged with fall mitigation deal with them. They always are placed on ranged, so you should never touch one.
The only thing we really have to worry about this phase is his knockback ability. Sometimes you resist it and stay where you are, other times you get knocked back into the middle of the platform. This is dangerous because it can knock you into the remenants of Sulfuras Smash or - rarely - into the waves themselves. To avoid the first pitfall, just move/angle a bit to the side after he smashes. To avoid the second, make copious offerings to the luck gods.
At 70% Ragnaros hits his first transition phase.
Three things will always happen in this phase. 1/3 of the platform will engulf in flames. Sulfuras Smash will drop. Searing Seeds will plant themselves where raid memembers stand. These three things may seemingly come at random, but there's actually a set pattern.
After the transition phase everyone should stack to one side. Immediately after Sulfuras Smash hits, Molten Seeds will plant themselves where everyone is. The exact second they appear everyone must run to the opposite side of the platform, because the seeds grow and do a distance-based AOE explosion when they blow-up. In 10man it's easier to be standing far enough away from each other, yet still firmly on the side, to avoid two seeds coming down on each other. In 25m you have to be a little more careful that you spread enough to ensure the seeds (which do distance based damage before they explode) don't kill those nearest, but remain close enough to the side that you don't have exploding seeds reach you when you shift. Melee is especially infamous for causing role-wide death with seeds. We like to stack on each other, it generally means we're in the right place. But having six melee stacked with seeds dropping = dead melee and angry raid leader. Spread out! Rag has a GIANT hit box so you can pretty far into the middle of the platform and still be hitting him.
After the seeds explode, little adds come out of them and converge on the raid. These need to be AOE'd down fast. I'm going to be honest here...I don't touch these adds. My AOE contribution is neligible and I lose a ton of DPS on the boss if I try to switch to them. The adds just die too fast to really make use of multiple flameshock + fire nova. Of course, I can really only get away with tunneling in on the boss because I have a solid group of other raiders that can pull the AOE needed to down the adds. If we struggled a bit more with DPS on them I would switch to them. Luckily that's not the case (and now with nerfs!...).
After the adds the floor will spontaneously combuts. This can happen in front, middle, or back. If it happens where you are standing, move. You can still hit Rag from the middle (just barely) and there is enough warning time between the charred, cracked floor visual and the actual floor-of-pain that I could run out of there backwards and still make it in time.
The 2nd shift will always have a Sulf Smash right after. Just know that on that 2nd shift you should all stack up front. I usually have shammy rage up when I can, and I drop stoneclaw at the beginning of every shift (before shifting into ghost wolf) for a bit of a buffer while I run.
At 40% you begin the second Transition. Remember your positions, they don't change from Transition A. Kill the little adds, just as in the first one. However, there are now two big adds on screen. They should get picked up/misdirected to the tanks and then ignored. Kill all the little adds but one and switch to the scions. Have ranged ready to kill that last little add when he gets too close to the hammer, but not before. This is so that you have max dps time on the scions without Rag up.
You might get Blazing Heat put on you. If you do, you should run to the outside of the platform and away from everyone else. This is both to 1) not light your raid on fire and 2) not heal the small adds with the fire, because it will.
When Ragnaros emerges your tanks should drag the remaining scion(s) to the middle. Once they are dead it's a full out DPS race on Ragnaros.
About 30seconds into this Phase the first meteor will come down. It WILL try to land on someone so make sure everyone is paying attention. If you get hit by a meteor in any fashion (it lands on you, it's fixated on you and catches up, you're standing in it's path, you sneeze near it) you will die. The meteors can be slowed, and upon damaging them (only when they're lit) they will be knocked back.
If everyone goes about hitting the meteors, they will be knocked back willy-nilly into the raid. This is bad. The person who the meteor fixates on should run from the middle to the back sides of the platform, once there the person angles themselves so that the meteor is knocked off the edge. From there on out the meteors new target should always be able to turn and knock it back out.
At 10% the fight is over! Congrats. You scared Ragnaros into hiding.Only on heroic will that last 10% mean anything.
Spoils of war: Matrix Restabilizer. DROOL. Choker of the Vanquished Lord .
The Blooper Reel:
1) Never have a priest lifegrip someone during smash. Biggest heart attack I have ever have. Died, ahnked, was immediately lifegripped into an oncoming wave. I lived, but it scared the crap out of myself and the other 5 healers who saw my health go from 100% to 2%.
2)If you're going to wipe anyway, make sure you go by smash. There's something satisfying about that giant hammer just smooshing you in one blow. Like some old school arcade game whack-a-raider.