A Hard 7×7 Puzzle Solved with Forced Chain in 20 Steps

This 7×7 board rates as hard (difficulty score 1071). Solving it from scratch takes 20 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 row 2, so its marker has to land there; that clears every other open cell in the row.
2. Next, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
3. Then, together, two regions have open cells only in columns G 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.
4. After that, every remaining candidate in this region touches A1 and B1 and A3 and B3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
5. Now, every remaining candidate in this region touches G4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, together, two regions have open cells only in rows 1 and 3; 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.
7. Following that, every remaining candidate in this region touches E5 and G5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
8. First, marking D1 would force F4 cleared (every remaining candidate in this region touches F4), then F5 (the last open cell in region 6), then C7 (the last open cell in region 5), and then region 7 would be left with no open cell for its marker. So D1 can't be the marker there.
9. Next, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
10. Then, every open cell in column D belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
11. After that, every remaining candidate in this region touches B2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
12. Now, 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.
13. From there, marking G1 would force C3 (the last open cell in region 1), then E6 (the last open cell in region 7), and then row 5 would be left with no open cell for its marker. So G1 can't be the marker there.
14. Following that, place a marker at C1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column C, and its region.
15. First, every remaining candidate in this region touches F4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
16. Next, place a marker at F5. Region has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region. Placing here also clears its 2 touching neighbours.
17. Then, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region.
18. After that, place a marker at E3. Region has exactly one open cell left. Placing here also clears its 1 touching neighbour.
19. Now, place a marker at D6. Region has exactly one open cell left. Placing here clears the rest of row 6, column D, and its region.
20. Finally, place a marker at B4. Region has exactly one open cell left.

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