A Hard 6×6 Puzzle Solved with Forced Chain in 17 Steps

This 6×6 board rates as hard (difficulty score 1075). Solving it from scratch takes 17 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 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 row 1, 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 3 and 4; 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 remaining candidate in this region touches D1 and D3, 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 B3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, that one can't be the marker: at C1 it would force E2 (the last open cell in region 4), and region 3 would be left with no open cell for its marker.
7. Following that, 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.
8. First, every remaining candidate in this region touches C3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
9. Next, if E1 were the marker, it would force C2 (the last open cell in region 4), then F3 (the last open cell in region 3), then D6 (the last open cell in region 5), and the chain continues, and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, 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.
11. After that, every remaining candidate in this region touches D5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
12. Now, if C2 were the marker, it would force E3 (the last open cell in region 3), then D6 (the last open cell in region 5), then A5 and B5 cleared (every remaining candidate in this region touches A5 and B5), and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
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 1 touching neighbour.
14. Following 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. Placing here also clears its 1 touching neighbour.
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. Next, 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.
17. Finally, place a marker at E5. Region has exactly one open cell left.

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