Here is the full list of all 28 The Simulacrum achievements.
Won Your First Game
Reach Floor 10
Unlock Sorla's Boss Battle
Unlock The Tortoise's Boss Battle
Reach Floor 50
Defeat Sorla's Boss Battle
Defeat The Underworld Tortoise's Boss Battle
Unlock Radas's Boss Battle
Unlock Zarek's Boss Battle
Defeat Radas's Boss Battle
Defeat Zarek's Boss Battle
Unlock Lilith's Boss Battle
Defeat Lilith's Boss Battle
Unlock Zafar's Boss Battle
Defeat Zafar's Boss Battle
Defeat Polyphyras' Boss Battle
Unlock Curious Colossus' Boss Battle
Defeat Curious Colossus' Boss Battle
Unlock Rothol's Boss Battle
Defeat Rothol's Boss Battle
Unlock Hyroc's Boss Battle
Defeat Hyroc's Boss Battle
Unlock Robert's Boss Battle
Defeat Robert's Boss Battle
Unlock Illus' Boss Battle
Defeat Illus' Boss Battle
Unlock The Arbiter's Boss Battle
Defeat the Arbiter's Boss Battle