An Expert 6×6 Puzzle Solved with Forced Chain in 18 Steps

This 6×6 board rates as expert (difficulty score 1144). Solving it from scratch takes 18 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, 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 remaining candidate in this region touches B2, 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 C5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, marking A1 would force B5 and B6 cleared (every open cell left in this region sits in column B, so its marker has to land there), then D3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then F3 and F4 cleared (together, two regions have open cells only in rows 3 and 4), and the chain continues, and then column E would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, if B1 were the marker, it would force D3 cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then E3 cleared (every remaining candidate in this region touches E3), then F3 (the last open cell in row 3), and column E would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, that one can't be the marker: at C1 it would force F2 (the last open cell in row 2), then B3 (the last open cell in row 3), and column A would be left with no open cell for its marker.
7. Following that, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
8. First, every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
9. Next, 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.
10. Then, every remaining candidate in this region touches B5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
11. After that, if E1 were the marker, it would force F3 (the last open cell in row 3), then A4 and B4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), then B6 (the last open cell in column B), and row 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, that one can't be the marker: at F1 it would force E3 (the last open cell in row 3), and region 5 would be left with no open cell for its marker.
13. From there, place a marker at D1. Region has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region. Placing here also clears its 1 touching neighbour.
14. Following that, 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.
15. First, place a marker at B4. Region has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region.
16. Next, place a marker at E3. Region has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region.
17. Then, place a marker at C6. Region has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region.
18. Finally, place a marker at F5. Region has exactly one open cell left.

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