A Hard 6×6 Puzzle Solved with Forced Chain in 16 Steps

This 6×6 board rates as hard (difficulty score 1032). Solving it from scratch takes 16 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 6, so its marker has to land there; that clears every other open cell in the row.
2. Next, every remaining candidate in this region touches E4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches A5 and B5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
4. After that, marking A1 would force B6 (the last open cell in region 6), then C3 (the last open cell in region 4), and then region 2 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, every remaining candidate in this region touches C1, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
6. From there, 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.
7. Following that, if B1 were the marker, it would force A6 (the last open cell in region 6), then C3 (the last open cell in region 4), and region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
8. 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.
9. Next, every remaining candidate in this region touches B3 and C3, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
10. Then, together, two regions have open cells only in columns A and B; 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.
11. After that, place a marker at C2. Region has exactly one open cell left. Placing here clears the rest of row 2, column C, and its region. Placing here also clears its 2 touching neighbours.
12. Now, place a marker at E1. Region has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region.
13. From there, place a marker at D4. Region has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region.
14. Following that, 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.
15. First, place a marker at F5. Region has exactly one open cell left.
16. Finally, place a marker at B6. Region has exactly one open cell left.

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