I did 5 dungeons on five characters, separately, not in the same group, for the weekly TW quest. After the 5th char, i got the achieve and the mount. Definitely not how it is supposed to work, as the achieve says for 5 weeks.
To be clear, I did each char separately, not the cheese way with 5 accounts in one group.
I got it on a single alt just doing 5 additional dungeons after I already had the mastery of the timeways buff
yep i got it lvling alt
it seems to be that instead of limiting it to once a week, every time you refresh the buff by doing a dungeon it counts towards the achievement. so if you do 9 dungeons as you level, you get the achievement