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

This 7×7 board rates as hard (difficulty score 1046). 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 C, so its marker has to land there; that clears every other open cell in the column.
2. Next, every remaining candidate in this region touches B4 and D4 and B5 and D5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
3. Then, marking A1 would immediately leave region 3 with no open cell for its marker, so it can't be the marker there.
4. After that, every open cell in column A 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.
5. Now, together, two regions have open cells only in columns B and D; 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.
6. From there, if E1 were the marker, it would force A2 (the last open cell in row 2), then D3 (the last open cell in region 1), then F5 (the last open cell in region 4), and row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
7. Following that, that one can't be the marker: at F1 it would force A2 (the last open cell in row 2), then D3 (the last open cell in region 1), then E5 (the last open cell in region 4), and column C would be left with no open cell for its marker.
8. First, marking G1 would force A3, E3 and F3 cleared (every open cell left in this region sits in row 3, so its marker has to land there), then C4 cleared (every remaining candidate in this region touches C4), then C5 (the last open cell in region 5), and the chain continues, and then region 4 would be left with no open cell for its marker. So G1 can't be the marker there.
9. Next, place a marker at B1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region. Placing here also clears its 1 touching neighbour.
10. Then, 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. Placing here also clears its 3 touching neighbours.
11. After that, place a marker at C4. Region has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region.
12. Now, 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.
13. From there, place a marker at E3. Column 5 has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, place a marker at G2. Region has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region.
15. First, place a marker at F7. Region has exactly one open cell left. Placing here clears the rest of row 7, column F, and its region.
16. Finally, place a marker at A5. Region has exactly one open cell left.

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