I attempted this several times with my group of 5 shammy. The thing that got me was the heal. If a heal got off then I was both in danger of one of my guys being bursted down or me running out of mana. I never did kill him but I'm pretty confident I could if I had an enemy castbar mod, several times I got them very very close to dead. I didn't get to phase2, but I imagine it would be pretty easy, worst case probably would require a mana pot.

I wasted time when I turned on castbars underneath nameplates, but didn't realize it still only showed that for your current target (/fail blizzard).