Microsoft Jigsaw

The description is a bit misleading; you actually need to finish all 6 difficulty stars on 10 puzzles to unlock this. The quickest way to do this is to download the "Kids" pack. Each of the 10 puzzles' last difficulties are only 190ish pieces vs 600 for other packs.

This can be done on the Kids Pack, which has 10 puzzles. The maximum number of pieces for a puzzle is 196 for the Grandmaster puzzles in the pack. And rather than solve each puzzle manually, you can use the following trick from the guide for "Great Expert":

1.) Open up the puzzle.
2.) Lock the view.
3.) Before making any move, highlight all pieces.
4.) Grab one of the distinct corner pieces of the puzzle (I've confirmed it doesn't always have to be the lower right one) and move the whole group of pieces to the opposite corner of the screen, so that none of them are visible. If, on one corner area of the puzzle, you can't find a distinct corner piece, then try another corner and use the corner opposite to it to move the pieces into a hidden position. If this happens for all four corner areas of the puzzle, then you can move a piece in the corner area so that it completes a corner of the puzzle, highlight all pieces and grab onto the group by that piece. This will only lead to 1 misplaced piece that need to be placed manually.
5.) Now let go of the group of pieces. If done right, they'll all form one group.
6.) Highlight the group of pieces and move it to the upper-left or mid-left part of the board.
7.) Grab the top piece of the stack, lower it about the length of one piece and let go. If done correctly, it should join to the piece that was previously below it. Now grab the top of the joined group of pieces (2 pieces if it's the second time) and lower it again.
8.) Repeat #7 until you form a column of the puzzle. Set this column aside or connect it to another column if you already have it and start a new column using #7.
9.) Repeat #8 until you've completed the puzzle. Alternatively, you can also click and drag the whole solved portion of the puzzle and move it so its edge meets the group of pieces formed in #5 from the the bottom to the top, allowing you to print the puzzle in columns from the right to the left.

This works for all of the difficulties and will allow you to quickly get all six difficulty stars done for each puzzle.

The other guide mentioning the kids pack is spot-on. The problem is that the kids pack (and all your other purchased content) tends to go missing.

To get the kids pack back, go to settings (Windows settings, not the game) -> Apps -> Apps & Features. Scroll down to Microsoft Jigsaw, click it, click Advanced Options, then scroll down and click 'reset'. When you open the game, it'll be as if you opened it for the first time, but it'll remember what you've completed and purchased. Re-download the kids pack (under 'see all collections') and you are all set. It'll disappear again in the future, though, so be prepared to do this multiple times while you work on the achievement.