A Hard 7×7 Puzzle Solved with Forced Chain in 16 Steps

This 7×7 board rates as hard (difficulty score 1005). Solving it from scratch takes 16 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
2. Next, together, two regions have open cells only in columns F and G; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
3. Then, every remaining candidate in this region touches A6 and C6 and A7 and C7, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
4. After that, marking A1 would force C5 (the last open cell in column C), then B7 (the last open cell in region 7), and then column F would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, that one can't be the marker: at C1 it would force F7 (the last open cell in column F), then A5 (the last open cell in region 5), and column B would be left with no open cell for its marker.
6. From there, together, two regions have open cells only in columns D and E; with one marker each, those two markers have to fill exactly those two columns, so every other region's open cells there can be cleared.
7. Following that, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
8. First, together, two regions have open cells only in rows 6 and 7; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
9. Next, every remaining candidate in this region touches B6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
10. Then, place a marker at B7. Region has exactly one open cell left. Placing here clears the rest of row 7, column B, and its region.
11. After that, place a marker at G6. Region has exactly one open cell left. Placing here clears the rest of row 6, column G, and its region.
12. Now, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region. Placing here also clears its 1 touching neighbour.
13. From there, place a marker at D2. Region has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region. Placing here also clears its 2 touching neighbours.
14. Following that, place a marker at E4. Region has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region.
15. First, place a marker at A3. Region has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
16. Finally, place a marker at C5. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎