Done the entire "Champions" achievement line months ago. Yesterday I have done the story, done the Dragonstorm one time in Public mode, it didn't count. Done it for a second time today, and I have hit both dragons, it still didn't count........... This is an absurd and it is frustrating players!!!
At least come with a red label pop-up disclaimer about how should it be done before entering the instance.... How are we supposed to know how to get the achievement!? You are setting up faulty expectations to people!