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
Mince Pies Puzzle Complete!
Dawn Puzzle Complete!
Toy Santa Puzzle Complete!
Bridge Puzzle Complete!
Baubles Puzzle Complete!
Bells Puzzle Complete!
Santa Puzzle Complete!
Nut Cracker Puzzle Complete!
Taken Puzzle Complete!
Snowman Puzzle Complete!
Teddy Puzzle Complete!
Gnome Puzzle Complete!
Cat Puzzle Complete!
Popcorn Puzzle Complete!
Gift Puzzle Complete!
Apples Puzzle Complete!
Friends Puzzle Complete!
Socks Puzzle Complete!
Cake Puzzle Complete!
Father Christmas Puzzle Complete!
Complete Santa Hat
Presents Puzzle Complete!
Bear Puzzle Complete!
Nativity Puzzle Complete!
Cookies Puzzle Complete!
Merry Christmas Puzzle Complete!