The achievement calls for you to kill all of the 10-man bosses and all heroic bosses present in Wrath of the Lich King at launch. The bug is in the fact that while Malygos isn't listed as a required boss kill, it really is. If you kill Maylgos, along with having killed every other 10-man and 5-man boss, you'll get the achievement.
Tigole originally announced this bug back on November 3rd, but it really hasn't been coming into play until now. We've received a few tips and complaints about it, and thought it worthy to bring to your attention.
We're expecting this to be fixed in patch 3.0.4. While no official announcement has been made, with the numerous announcements of 3.0.4 changes last week we're expecting the PTR for patch 3.0.4 to drop soon – if not this week.