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