It was a one time event. That it has an achievement is a mistake. Only permanent/recurring things should have achievements. We can't remove an achievement once it's gone live, but I am having QA assign me a task to investigate if I can possibly change the requirements so that it is attainable. There's no timetable on this atm, but if such a change is possible and doesn't cause any bugs, I'll try to get it in in the next few months.
Replies
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/comment/1145328/#Comment_1145328