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.
Score over 10000 points in the balloon minigame.
Score over 10000 points in the car minigame.
Score over 10000 points in the boat minigame.
Find all bones.
Complete the story with the score over 42000 points.
Complete the masterpiece.
Complete the story under 20 helps.
Complete winter season and defeat the Ice Giant.
Complete autumn season.
Complete summer season.
Complete spring season.
Complete first chapter.