Here is the full list of all 11 Back To Ashes achievements.
Defeat Katos.
Defeat Stone Golem.
Defeat Oron, Disciple of Bones.
Defeat Lucas, the Forsworn Titan.
Defeat Alba, Disciple of Devotion
Defeat To'Has the Guardian
Defeat Giant Crab.
Defeat Vhaadzaguq.
Defeat The First
Defeat Angar, Ash Collector.
Defeat Inwaer The Last