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