Here is the full list of all 22 Adventuring Gentleman achievements.
Defeated mechanical Sofia
Defeated mechanical God
Looked at Dorian's Portrait
Defeated King in Yellow
Defeated Steam Dragon
Defeated Count in Yellow
Fetched the Sun
Freed Canterville Ghost
Defeated Battle Automoton
Caught Flower Fairy
Defeated Nutcracker King
Defeated Goblin Wizard
Defeated Ornithopter
Turned off Sired
Defeated Artilerist
Defeated clockwork Sofia
Defeated Many Eyed Moon
Defeated Selfish Giant
Defeated Diving Suit
Defeated King of the Mushrooms
Defeated Goblin Gunner
First enemy killed