How to solve this 6×6 logic puzzle

6×6 · severe · 19 steps

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This 6×6 board rates as severe (difficulty score 1156). Solving it from scratch takes 26 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.

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.
Next, every remaining candidate in this region touches A3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
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.
After that, every remaining candidate in this region touches D4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Now, marking A1 would force F2 (the last open cell in row 2), then C6 (the last open cell in region 6), and then column D would be left with no open cell for its marker. So A1 can't be the marker there.
From there, if B1 were the marker, it would force A4 (the last open cell in region 3), then C6 (the last open cell in region 6), and column D would end up with no open cell for its marker, which can't happen. So it can't go there.
Following that, that one can't be the marker: at C1 it would force D5 (the last open cell in region 4), and column E would be left with no open cell for its marker.
First, 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.
Next, 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.
Then, marking D1 would immediately leave column F with no open cell for its marker, so it can't be the marker there.
After that, if E1 were the marker, it would force D5 (the last open cell in column D), then B3 (the last open cell in row 3), and row 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
Now, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
From there, every open cell in column E 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.
Following that, every remaining candidate in this region touches D5, 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 D2. Column 4 has exactly one open cell left. Placing here clears the rest of row 2, column D, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at B3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column B, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region.
After that, place a marker at A6. Region has exactly one open cell left. Placing here clears the rest of row 6, column A, and its region.
Finally, place a marker at E4. Region has exactly one open cell left.

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