Zone: The Library: The Stuff of Dreams Mob: a furious sweeper Achievement: Witch Mount? Expected Results: Killing the 3 'a furious sweeper' mobs at the start of the mission, which count as 3 Broom Minnons kills. Observed Results: They do not count for this achievement.
I think I recall these not working either when I did it awhile back. These are the brooms summoned by the boss right?
IIRC this only applies to the ones in Arx Mentis. (also why it is listed as "optional" for the main reward)