Can we get an armory link? Are you using resistance+mastery elixirs or just stamina flasks? What healers are you using? I think killing the purple in some situations is possible, for example when you got BL/BK/P/Y, but you gotta time those mana CDs and actual healing CDs right.
Also, if your healers are amazing, green + yellow will be possible to be healed through. And by the way, I'm pretty sure that killing purple is absolutely viable at the P/G/BK/BL combo. But again, you will need healing CDs.
Personally, I attempted tanking it at the first week the heroics were available and last week by using the same priority you are using at the moment. To be honest, if you are on the ball with your CDs, your healers are very good and you also have a bit of luck, it's doable. However, we decided to tank it with a DK since it simplifies it so much.