You should try it with 4 shamans. It's so trivial. Just spam chain heal when he grabs the tank and dont worry about whom is he aggroing.

I usually place my shamans on the stairs and tank him in the middle of the room. Have to deal with the cloud crap he does, but every time I have tried to tank him against the wall he resets after throwing my tank or tank gets stuck in the wall.

BTW kudos for doing it with your team. I thought it was not possible without stacking lots of extra massive aoe healing like chain heal x 4 or 4 holy nova.