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