I do it that way too, just keep your distance from the rider, he wont aggro you if you stay back.

The dragon seemed to fly in and out of range from my group, so when he's in range, DPS him, when he's out of range, just hold your ground and keep those Grounding Totems up, if you have those down, you take 0 damage until he lands.

At around 35-40%, he lands, but he wont aggro the rider (at least, he never did with me, you sure you're not hitting him with chain lightning?). Then just hit Elemental Mastery/Trinket and kill him as quickly as possible, if you can, try to have whichever shaman he has targetted run forwards and to the side a bit, that way you can save the rest of your group from eating a fire breath attack.

Or, just buy some Greater or Major Fire Protection Potions, that's what I did at 61 to down him.

The second boss of Blood Furnace however, I can't do, will have to try again at 63.