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