Here is the full list of all 21 Mandew vs the Colorless Curse achievements.
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Complete a level without using the 'run' button.
Find the hidden power-up in level 1-1.
Clear a level start to finish with the same full power.
Roast a falling apple with a fireball.
Hit 10 enemies without touching the ground.
Defeat a bomb with an explosion.
Defeat a witch with a block from underneath.
Bribe Tero with food.
Take a mallet to Nevux's head.
Defeat Syax without taking a hit!
Reflect Recolor's wave with the hammer.
Defeat LengthenedMudTea.
Register scores for 1-1 to 1-4.
Register scores for 2-1 to 2-4.
Register scores for 3-1 to 3-4.
Register scores for 4-1 to 4-4.
Unlock the secret level.
Find a secret exit in any stage.
Complete race mode.
Try pressing F5 or F6...