Here is the full list of all 23 Figment achievements. It takes around 5-6 hours to unlock all of the achievements on Windows.
Complete the game
Defeat the Nightmare in the Pathway
Earn third bonus health point
Remove all Tentacles in the Pathway
Collect the memory in the Pathway
Defeat the Nightmare in Clockwork Town
Earn second bonus health point
Remove all Spiders from Clockwork Town
Defeat the Nightmare in Freedom Isles
Remove all Plague Rats from Freedom Isles
Earn first bonus health point
Kill 2 enemies at once with a heavy attack
Collect one memory
Knock on all doors in Outer Cerebrum and Cerebrum City
Complete the intro
Hit 3 enemies at once with heavy attack
Collect half the memories
Collect all the memories
Collect the Kettle Brothers' memory
Collect the light bulb houses' memory
Knock on all doors in Freedom Isles
Knock on all doors in Clockwork Town
Go back to Freedom Isles and Clockwork Town after defeating their nightmares