I just completed it.. this is how I did it.
1) Replay Story 5. The Missing Facet
2) Go to Story Instance and talk with Mabon
3) Exit Instance after you are done
4) Go to Meta Event and don't really do it, just kill some mobs so you get a participation (I did it only by spamming few fireballs with my Skyscale)
5) After the Meta is succesfull you get your achiev