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