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

This 7×7 board rates as hard (difficulty score 1020). Solving it from scratch takes 16 logical steps, using 8 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line 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 G, so its marker has to land there; that clears every other open cell in the column.
2. Next, every open cell in row 7 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
3. Then, together, two regions have open cells only in rows 1 and 2; 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.
4. After that, together, two regions have open cells only in columns E and F; 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.
5. Now, every remaining candidate in this region touches F3 and F4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
6. From there, marking A1 would force F2 (the last open cell in region 3), then G4 (the last open cell in region 4), then E6 (the last open cell in region 6), and then row 5 would be left with no open cell for its marker. So A1 can't be the marker there.
7. Following that, if B1 were the marker, it would force F2 (the last open cell in region 3), then G4 (the last open cell in region 4), then E6 (the last open cell in region 6), and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
8. First, that one can't be the marker: at C1 it would force F2 (the last open cell in region 3), then G4 (the last open cell in region 4), then E6 (the last open cell in region 6), and region 5 would be left with no open cell for its marker.
9. Next, place a marker at A2. Region has exactly one open cell left. Placing here clears the rest of row 2, column A, and its region. Placing here also clears its 1 touching neighbour.
10. Then, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
11. After that, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region. Placing here also clears its 1 touching neighbour.
12. Now, place a marker at D5. Row 5 has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region. Placing here also clears its 1 touching neighbour.
13. From there, place a marker at C3. Region has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region.
14. Following that, place a marker at F6. Region has exactly one open cell left. Placing here clears the rest of row 6, column F, and its region.
15. First, place a marker at E1. Region has exactly one open cell left.
16. Finally, place a marker at B7. Region has exactly one open cell left.

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