Ok, I finally was able to do this today (Heroic)
....after finally sending my ISboxer config file over to Lax, we determined that there was a misconfigured setting that was interfering with sending the command to the other toons.

In a little over 3min I downed Ley using the dismounted healer tactic and we all got the "Amber Void" & "Emerald Void" achievements too!