Here is the full list of all 16 Savage: Ultimate Boss Fight achievements.
Defeat the Demiurge.
Defeat the Ifrit.
Defeat the Garuda.
Defeat the Titan.
Defeat the Leviathan.
Defeat the Ramuh.
Defeat the Shiva.
Defeat the Ifrit (Extreme).
Defeat the Garuda (Extreme).
Defeat the Titan (Extreme).
Defeat the Leviathan (Extreme).
Defeat the Ramuh (Extreme).
Defeat the Shiva (Extreme).
Defeat the Percival.
Defeat the Seraphim.
Defeat the Fafnir.