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