Get a full nine charges of Lightning Shield off of Ragnaros before he goes downstairs. As soon as your main target pops up, hit it with a Flame Shock, Lava Burst. Mid Lava Burst cast, tab targets to your secondary Son of Flame. Queue a mix of Lightning Bolts and Chain Lightnings dependent on how much time you need to take up till you can hit it with your nine-stack Fulmination. Or you could get the nine charges, Unleash Elements Ragnaros before he goes downstairs. As soon as your main target pops up, hit it with a Lava Burst followed by your nine-stack Fulmination. Depends on how much you're responsible for. If it's just the single add, do the latter. If you have to keep an eye on two, the former.
Re-edit: Heh, I decided to edit in text to be actually helpful and when I finish, I find Endus has the exact same plan.
Thunderstorm is huge. If you're having trouble keeping them off the hammer, a Thunderstorm at the right point can catch 3-4.
I usually try and charge up a Fulmination during the changeover; I won't fire it at Rag after he drops the hammer, I'll save it. I usually FS, LvB one that's close, and then fire an LB at it or another, and Fulmination on one that's rushing the hammer.
You should have some decent divisions of DPS, too. if people don't know which adds are "theirs", and which side they're working on, you're gonna wipe. A lot. Until you assign that stuff.
I precast earthquake in the path of elementals. Sometimes I'll get one or even two knockdowns on them. (You aren't doing anything else while the fireballs go to the spawn of the sons of flame.)
I only use TStorm if they are close enough to the hammer. Otherwise a Lava Burst and a FS can burn them down fast. Also a good idea to use CL here as well. Might catch the others helping your group.