Here is the full list of all 18 Hero's journey achievements.
Win the game on very easy difficulty
Win the game on easy difficulty
Win the game on hard difficulty
Win the game on very hard difficulty
Win the game on impossible difficulty
Kill Boss AmbleGuy
KillBossFootBoos
KillBossBowBoss
KillBossWarChief
KillBossLichBoss
KillBossDemon
KillBossUndeadKnight
ImpossibleKing
ImpossibleKnight
ImpossibleScientist
ImpossibleWarrior
ImpossibleWinterman
ImpossibleLina