To earn this achievement, you must avoid the Chaos Spawn from using their
Devourer ability, which consumes a cultist when the Chaos Spawn is injured enough. To avoid this, simply defeat all the cultists before targeting the Chaos Spawn. Once all the cultists are defeated, the Chaos Spawn is unable to use
Devourer and can be defeated, earning the achievement.
Along with
Aviation Saviour, this achievement can be made easier by lowering the difficulty until Aurora is close to death, and then returning the difficulty to Daring before delivering the finishing blow.