So when patch 6.0.3 dropped this achievement was removed for killing Heroic: Ra-den - Achievement - World of Warcraft
Ra-Den on heroic Difficulty. Blizzard is aware of the bug and plans to fix it when WoD goes live... So get your Achievement before Wed night.
Activity Feed - Community - World of Warcraft
The above link is to show you a friend of mine recieved the kill and its in their FoS.