A Hard 6×6 Puzzle Solved with Forced Chain in 11 Steps

This 6×6 board rates as hard (difficulty score 941). Solving it from scratch takes 11 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 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 B2 and D2 and B3 and D3, 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 force B6 (the last open cell in region 4), and then region 5 would be left with no open cell for its marker. So A1 can't be the marker there.
4. After that, marking D1 would force C3 (the last open cell in region 3), then B5 (the last open cell in region 5), and then column A would be left with no open cell for its marker. So D1 can't be the marker there.
5. Now, together, two regions have open cells only in columns B and A; 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, 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 1 touching neighbour.
7. Following that, place a marker at E4. Region has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region. Placing here also clears its 2 touching neighbours.
8. First, place a marker at A5. Row 5 has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region.
9. Next, place a marker at B1. Region 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 F2. Region has exactly one open cell left.
11. Finally, place a marker at C3. Region has exactly one open cell left.

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