Here is the full list of all 29 The Painter's Apprentice achievements.
Finish the game.
Unlock all gold stars.
Complete World 1
Complete World 2
Complete World 3
Complete World 4
Complete World 5
Complete World 6
Complete World 7
Complete the game by only killing bosses.
Unlock all encyclopedia entries.
Reach the ending without dying.
Defeat 10 blobs in less than 30 seconds.
Read all letters.
Achieve 100% completion.
Defeat two Psychedelic blobs in one stroke.
Defeat enemies using all three colors in under 10 seconds.
Unlock the secret level.
Die more than 10 times in a single level.
Defeat all Speedy blobs using only melee attacks.
Discover the truth behind it all.
Get the gold star on your first try for all levels.
Find all collectibles.
Win a boss fight without taking any damage.
Win all boss fights without dying.
Reach the ending without exploding any Unstable blobs.
Defeat 100 enemies.
Defeat 500 enemies.
Defeat 1000 enemies.