An Expert 7×7 Puzzle Solved with Forced Chain in 21 Steps

This 7×7 board rates as expert (difficulty score 1127). Solving it from scratch takes 21 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 remaining candidate in this region touches E2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
2. Next, 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.
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, every open cell in column G 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 rows 4 and 5; 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.
6. From there, marking A1 would force F2 (the last open cell in region 2), then G7 (the last open cell in row 7), then C4 cleared (every remaining candidate in this region touches C4), and the chain continues, and then column D 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 2), then C6 cleared (every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column), then C4 and D4 cleared (every remaining candidate in this region touches C4 and D4), and the chain continues, and row 4 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 2), then D3 cleared (every open cell left in this region sits in column D, so its marker has to land there), then B3 (the last open cell in region 4), and the chain continues, and column D would be left with no open cell for its marker.
9. Next, 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.
10. Then, marking D1 would force C4 (the last open cell in region 6), and then column F would be left with no open cell for its marker. So D1 can't be the marker there.
11. 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.
12. Now, every remaining candidate in this region touches C2 and C4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
13. From there, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
14. Following that, every remaining candidate in this region touches B3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
15. First, 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. Placing here also clears its 2 touching neighbours.
16. 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.
17. Then, place a marker at D5. Region 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.
18. After that, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region.
19. Now, place a marker at E1. Region has exactly one open cell left.
20. From there, place a marker at B6. Region has exactly one open cell left. Placing here clears the rest of row 6, column B, and its region.
21. Finally, place a marker at G7. Region has exactly one open cell left.

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