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