Well, I suppose it helps to have more then one class that can build aggro and tank for phase 1.

I've done her with my four elemental shaman by just switching spots with whomever overaggroes, and using some extra macro's to make sure the one that has aggro does most to keep aggro the longest either, as to not make it one big mess of constant aggro changes. And found her pretty easy. But phase one will no doubt cause an aggro wipe with a single healer present. The knockbacks onyxia does take away a fair bit of threat on your tank.

That said tho, I find it hard to believe Onyxia two shots a discipline priest. In fact, discipline priests shield be quite viable at being able to tank onyxia. :P So maybe stock up on some extra stamina gear for your priest just for that encounter?