The legacy achievement was briefly obtainable by obtaining a Tier 2 helm item from Nefarian in Blackwing Lair (previously a drop from Onyxia). This was corrected in patch 3.3.0.
Despite the previous correction, the achievement was still able to be obtained by completing the quest [60R] The Only Prescription, a part of a chain quest for creating the Scepter of the Shifting Sands, which previously required killing the original Onyxia (now requires killing the level 80 Onyxia). You can also get the Legacy achievement by completing the quest [60R] A Proper String, which requires looting a [Mature Black Dragon Sinew] (previously only reliably gathered from Onyxia, it now has a low chance to drop from lesser black dragons). These quests, along with [60R] The Forging of Quel'Serrar,  Victory for the Horde and  Victory for the Alliance were removed in Cataclysm, so although it was possible for a time to earn the legacy achievement after the removal of level-60 Onyxia's Lair, it is no longer so.