How to solve this 6×6 logic puzzle

6×6 · sharp · 17 steps

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This 6×6 board rates as sharp (difficulty score 1087). Solving it from scratch takes 24 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.

First, every open cell in column A 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.
Next, 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.
Then, marking A1 would force D2 (the last open cell in region 3), then F3 (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.
After that, that one can't be the marker: place it at C1, and region 3 would be left with no open cell for its marker.
Now, 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.
From there, together, two regions have open cells only in rows 2 and 3; 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.
Following that, 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.
First, if E1 were the marker, it would force C2 (the last open cell in row 2), then F3 (the last open cell in region 4), then D6 (the last open cell in region 6), and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
Next, that one can't be the marker: at F1 it would force C2 (the last open cell in row 2), then E3 (the last open cell in region 4), then D6 (the last open cell in region 6), and column B would be left with no open cell for its marker.
Then, 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.
After that, place a marker at F2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
Now, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
From there, every remaining candidate in this region touches C4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Following that, place a marker at A4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region. Placing here also clears its 1 touching neighbour.
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.
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.
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. ∎