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