Here is the full list of all 12 Axel & Pixel achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Complete spring season.
Complete first chapter.
Complete summer season.
Complete autumn season.
Complete winter season and defeat the Ice Giant.
Complete the masterpiece.
Find all bones.
Score over 10000 points in the balloon minigame.
Score over 10000 points in the car minigame.
Score over 10000 points in the boat minigame.
Complete the story under 20 helps.
Complete the story with the score over 42000 points.