You got some decent cc there, why don't you use it?
And why do you have troubles with the start of phase 2? Everyone is topped off before and unless all fail at the pillars there shouldn't be much more then 4-6 magma stacks average plus some shadow barrages to heal through.
We kited them with a frostmage on 10s, too. But he died from time to time.. You can try use your boomkin or your ret pally to actually tank/kite them with the aid of a frosttrap and a ring of frost it's very manageable. You just need to find a position where neither both dragon's tail/breath nor Onyxia's electro thingy can hit them.