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