Ashes, Ashes...

We just did this achievement with 6 hunters, you'll really want to bring as many as possible.

I've created a weak aura which tracks the ashes you've collected and writes the player who collected them in the chat.
You can get the weak aura here:
http://pastebin.com/9isYH54D

I'm amazed no ones posted this yet. I've been in half a dozen groups for this as it's my last glory achievement besides legion raider, but they either fail from being too weak, getting horrible RNG or lacking the patience and just getting burned out. Gave it a shot last night and suffered through a dozen attempts... until someone had an epiphany.

Demon Hunters. Demon Hunters are absolutely OP at this achievement. If you have a strong tank, you can probably easily duo this achievement if you have a demon hunter. A few of us had DH alts, and after one dumb fail getting the hang of it, our second attempt changed this from one of the hardest to one of the easiest glory achievements in the game. With 4 DHs, we probably finished this in about 30 seconds. You can either jump normally and use Vengeful Retreat to get back, but honestly double jumping and gliding back is the way to go. Use the NE and SW corners and play it smart to avoid going too far off, but generally it's very hard to fail this if you know how to jump. Even if someone has a level 100 DH fresh out of the starting zone, they will still contribute more to getting this achievement more than any other class at 110 can, and so it's very worth having your friends/raid switch to them if they have one.

Bloody Demon Hunters.

The ashes that needs to be collected can be found at the very edge of the platform in Phase 3. It is the "sparky" things falling from the ceiling. The way to collect them is for the ashes to pass through you, however that can be very difficult when they are not ON the platform.

So far we have managed to get 19 followed by a wipe. We sent hunters and warlocks aswell as a healer out for them to jump off and disengage/teleport back on. That way we can get a few atleast.

Still attempting to figure out a proper tactic.

EDIT: It would seem that this tactic of sending a few squads out to pick up ashes is very doable. However, in this case "the more the merrier" is very applicable. Being 30 people helps a lot. We sent out our hunters and a healer while we got Blackhand himself to ~2% and then we sent a wave of people to suicide for the remaining ashes and the remnants of the raid stood ready to finish off Blackhand, when the achievement criteria was fulfilled.

Best of luck!