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