Con-speed-atory (10 player)

A note for this achievement: The trash isn't tied to Freya, so you don't have to clear the entire room.

As for the encounter, Freya takes at least 6 minutes to kill (well more, but I didn't want to be nit picked) because of the structure of the encounter. Essentially you have 12-13 minutes to clear the necessary trash, the 3 elders (unless you plan to one shot part or all of her hard mode, which actually might make this easier) and pull Freya to a safe spot

EDIT: Some additions. As of the time of this edit, you can pull and kill the first Elder (fists of stone guy) without getting any trash and killing him doesn't start the timer. This can easily shave 1 minute or more.

Also, I got this simply by speed chain pulling. With no more than 15 or so seconds between the death of the last of the previous pack and the pull of the first of the next. It was a higher dps group and once we got to phase 2, we brought freya down in about 1 minute 10 seconds, and we had 10 seconds to spare. So with some luck and some surpreme chain pulling, this is doable without killing the first elder before trash, but it is probably harder that way

Important if you are doing it these days - you have to kill at least one trash mob before killing the boss, otherwise the timer won't start and you won't get the achievement.

Maybe obvious but I failed on that last week.

EDIT: The description has now been changed from "within 20 minutes of entering" to "within 20 minutes of first kill" making this comment obsolete.