I wonder if the „Kill the Hydra Queen“ achievement is still bugged, or bugged again? According to the wiki it was bugged till 4/15/14.
http://wiki.guildwars2.com/wiki/Admiral_Taidha_Covington
But in the wiki are bug reports after that date.
http://wiki.guildwars2.com/wiki/Talk:Kill_the_Hydra_Queen
I didn’t got the achievement a while ago and I killed Admiral Taidha Covington again at the weekend. I got gold participation in the event and a drop of “Mistward Coat Panel”
https://wiki.guildwars2.com/wiki/Mistward_Coat_Panel
so I must have tagged her but again no „Kill the Hydra Queen“ achievement. :-(