Here is the full list of all 27 Pixel Puzzles 2: Christmas achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Complete All Puzzles
Nut Cracker Puzzle Complete!
Gift Puzzle Complete!
Apples Puzzle Complete!
Nativity Puzzle Complete!
Cookies Puzzle Complete!
Popcorn Puzzle Complete!
Cat Puzzle Complete!
Santa Puzzle Complete!
Father Christmas Puzzle Complete!
Taken Puzzle Complete!
Snowman Puzzle Complete!
Friends Puzzle Complete!
Bear Puzzle Complete!
Socks Puzzle Complete!
Toy Santa Puzzle Complete!
Cake Puzzle Complete!
Presents Puzzle Complete!
Merry Christmas Puzzle Complete!
Teddy Puzzle Complete!
Bells Puzzle Complete!
Gnome Puzzle Complete!
Mince Pies Puzzle Complete!
Baubles Puzzle Complete!
Dawn Puzzle Complete!
Bridge Puzzle Complete!
Complete Santa Hat