Druids are the red-robed spellcasters found mostly in Hypno Baron's Castle, though after beating Hypno Baron one can also be found at the Mayor's Retreat (which is closer to a Warp Squid).
To get the achievement, you must kill one of them between the hours of 00:00 and 01:00 (or 12:00am-1:00am), according to your system's clock.