In the Court of Bones, summon and defeat one of the ancient mogu listed below.
The Mogu Below-gu
These Mogu Have Gotta Go-gu
This achievement is awarded for killing one of the three Undead summonable mobs on the Isle of Thunder who drop a Fortuitous Coffer containing Elder Charm of Good Fortune. These mobs are quite easy to solo, but you do have to avoid their attacks. Also note that while these mobs' health values will increase as more players attack, they do NOT have the loot sharing system enabled.