How to solve this 6×6 logic puzzle

6×6 · severe · 18 steps

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This 6×6 board rates as severe (difficulty score 1159). Solving it from scratch takes 24 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Line confinement, Paired regions, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, together, two regions have open cells only in columns E and F; 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.
Next, 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.
Then, marking A1 would force F2 (the last open cell in region 2), then E5 (the last open cell in region 4), and then column D would be left with no open cell for its marker. So A1 can't be the marker there.
After that, if B1 were the marker, it would force F2 (the last open cell in region 2), and column D would end up with no open cell for its marker, which can't happen. So it can't go there.
Now, that one can't be the marker: at C1 it would force F2 (the last open cell in region 2), and column D would be left with no open cell for its marker.
From there, that one can't be the marker: at F1 it would force D2 (the last open cell in row 2), then C6 (the last open cell in column C), then E5 (the last open cell in region 4), and row 4 would be left with no open cell for its marker.
Following that, that one can't be the marker: at A2 it would force E1 (the last open cell in region 2), then F3 (the last open cell in row 3), then B4 (the last open cell in region 5), and the chain continues, and row 6 would be left with no open cell for its marker.
First, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, if E1 were the marker, it would force B2 (the last open cell in row 2), then F3 (the last open cell in row 3), then A4 (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.
Then, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region.
After that, place a marker at D1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
Now, every open cell in row 3 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, every open cell in row 4 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.
Following that, every remaining candidate in this region touches B4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
First, place a marker at A4. Region 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.
Next, 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.
Then, 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. ∎