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