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