Here is the full list of all 20 A Gentlemanly Adventure achievements.
Putting it out of its misery
You fell down a hole.
Complete Act 1
Complete Act 2
Complete Act 3
Complete Act 4
Save the world, and return home.
Accidentally doom your reality.
Defeat the Starved Cave Troll
Defeat the Witch Doctor
Defeat the Giant Eagle
Defeat the Giant Lizard
Defeat the Demon
Defeat Gary, the Minotaur King
Defeat the Gryphon
Defeat the Cave Troll
Defeat the Abomination
Defeat the Lord of Flame
Defeat the Hermit
Defeat the Kraken