Thx a lot for your very constructive imputs. To summ up what most of you basically are saying is something like this:
Legs: Feral and enhance shaman along with the elemental shaman (so he can knock the obsidans away as well if they get too close to the boss).
Sparks: The two mages on sparks and when no sparks up on the legs.
Fragments: The hunter and lock take care of those.
And pop heroism after 4th spark is spawned and all nukes the boss to push into the 2nd phase. The paladin whos tanking should try to use Inquisition and Hammer of Righteous. The hunter use MM and the fire mage should instead go arcane.
Yes, that sounds like a better strat than whatever it was that you were doing that was taking so much focus off the boss - as most people have guessed, I would assume you were too worried about the little oozes. Your paladin should always try to tank the fragments on the legs so the legs get incidental cleaves as well as any oozes that are around. If your shaman happens to notice a bunch that are close then using a knockback is fine but you really, really don't need to worry about the oozes at all. Some will die from all the cleaving, and those that get through will not make a big difference at all if you are doing the fight right.
The other thing that may be hurting you is your driving. If you aren't driving him very well then not only will you get a lot more damage for your healers but your dps suffers as well since you lose the armor slower. Make sure the person doing it is someone with good raid awareness so they always know where the inactive and active volcanoes are (you shouldn't have to run over an inactive volcano very much at all) and that when there isn't an active volcano up you are always turning him towards the middle or towards a group of inactive volcanoes so that when he activates one you aren't out of position. And of course make sure that your raid is listening to your driver and not over-reacting to his calls. If you are still having trouble with that there are addons out there that can tell you who is targeting what leg so that you can make sure people are doing the right things.
Biggest flaw I see in your strat is you only have 2 dps on legs full time. That needs to be at least 4, and the other 3 on the adds. The other 3 need to help out on the legs as best as they can as well.