Kind of rethinking the format, see what you think:
Useful in non-scaling content (faction):
Alderfather (Primal Rift)
Chief Dargon (Fang Moor)
Copycat (Mirrored Halls)
Drowned Sailor (Sunken Fleet)
Ethereal Sentry (Hall of Guardians)
Gargoyle (Hall of Guardians)
Grave Seer (Crypt Keepers)
Harpy Mage (Stonesong Eyrie)
King Gobtruffle (Amanithrax)
Luna (The Warrens)
Necrocorn (Silver Necropolis)
Queen Xochi (Stonesong Eyrie)
The Deep King (Sea of Sorrow)
The Frostfire King (Frostfire Keep)
The Maraji Queen (Sunken Fleet)
The Wendigo (Wild Court)
Vanya Soulmourn (Silver Necropolis)
Water Elemental (Sunken Fleet)
Useful in scaling content only (faction):
Lady Morana (Crypt Keepers)
Sea Witch (Sea of Sorrow)
Silent Sentinel (Hall of Guardians)
Sledgepaw (Dark Pits)
Watch Mother (All-Seeing Eye)
EDIT: Aaand we’re up to date as of Frostfire Keep. Definitely erred on the side of inclusion with some of these, please let me know if you think I went too far.