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