I did it with 900 resto druid (which is my offspec). 30 attempts. Prydaz + Sephuz (since they're the only legendaries I have that work on resto).

If you're below ~890 then yeah you probably need more gear. But if not, then it's not actually a throughput challenge. You should spec into cooldowns that help with healing for short phases as opposed to throughput talents that help with mana. If you find yourself lacking healing, then the real problem is most likely that you're not using your cc correctly or you're not doing enough damage yourself.

Also, there is no one-shot. The arcane caster ramps up his damage and will eventually oneshot, but that's why you need to cc him. I suggest to stun him on his 4th cast. If you have extra cc you can use that on the blade dance, but that can be healed through anyway. If you do it this way, even 60% hp is safe for the little guys (I know this because I'm resto druid and I would only place hots if they were above 60%, which worked far better than constantly healing). If you don't have extra cc you will need to use some kind of healing cd for the 2x blade dance. I prehotted all 5 and used my artifact ability, which was enough.