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