Celestalon tweeted today that doing Proving Grounds Tank Gold as dps or healer awards you with the achievement right when 6.0 comes. Or any other gold spec that isn't yours.
http://wod.wowhead.com/achievement=8812
Ciaossu updated his post so you can see the tweet.
I was thinking about creating a new thread for this becausue demon lost a LOT of it's toughness on WoD. Passive stun/snare reduction is gone, and worse of all, no longer we have Dark Apotheoses.
I haven't tried gold tank on beta yet, but on live doing it without Dark Apotheoses I think it's impossible. You can't maintain aggro, you won't be able to tank nearly as much and so on.
My advice, if you want the FoS, do it NOW. It's a feat so I don't think Blizzard will be going easy on this one. Sparkuggz also has a video of him doing it.
I just finished it after one hour of trys, and it's somewhat brutal. It should be named "Kitting grounds".
http://postimg.org/image/ly88vmw5p/
Full Mastery.