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