How to solve this 8×8 logic puzzle

8×8 · severe · 27 steps

3🐱

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This 8×8 board rates as severe (difficulty score 1398). Solving it from scratch takes 37 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, place a marker at H1. Region has exactly one open cell left. Placing here clears the rest of row 1, column H, and its region. Placing here also clears its 1 touching neighbour.
Next, every remaining candidate in this region touches F3, 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 E4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, marking B2 would force E3 (the last open cell in region 2), then D5 (the last open cell in region 6), then A4 (the last open cell in region 5), and the chain continues, and then row 6 would be left with no open cell for its marker. So B2 can't be the marker there.
Now, that one can't be the marker: at D2 it would force F4 (the last open cell in region 2), and region 6 would be left with no open cell for its marker.
From there, if A3 were the marker, it would force B8 (the last open cell in column B), then G7 (the last open cell in column G), and row 6 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: place it at B3, and region 1 would be left with no open cell for its marker.
First, if D3 were the marker, it would force E5 (the last open cell in region 6), then F2 (the last open cell in region 2), and region 7 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 E3 it would force D5 (the last open cell in region 6), then G4 (the last open cell in region 4), then B6 (the last open cell in region 5), and row 7 would be left with no open cell for its marker.
Then, that one can't be the marker: at A2 it would force F4 (the last open cell in region 2), and row 3 would be left with no open cell for its marker.
After that, if C2 were the marker, it would force F4 (the last open cell in region 2), and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
Now, 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.
From there, if G3 were the marker, it would force E2 (the last open cell in region 2), then D5 (the last open cell in region 6), then B4 (the last open cell in region 1), and region 5 would end up with no open cell for its marker, which can't happen. So it can't go there.
Following that, place a marker at C3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region. Placing here also clears its 1 touching neighbour.
First, every open cell left in this region sits in row 5, so its marker has to land there; that clears every other open cell in the row.
Next, every remaining candidate in this region touches D6 and E6, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Then, every remaining candidate in this region touches F7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, every open cell left in this region sits in column G, so its marker has to land there; that clears every other open cell in the column.
Now, 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.
From there, place a marker at D5. Region has exactly one open cell left. Placing here clears the rest of row 5, column D, and its region.
Following that, marking E2 would force G4 (the last open cell in region 4), then B6 (the last open cell in region 5), and then row 7 would be left with no open cell for its marker. So E2 can't be the marker there.
First, 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.
Next, marking A4 would immediately leave row 6 with no open cell for its marker, so it can't be the marker there.
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. Placing here also clears its 1 touching neighbour.
After that, place a marker at A8. Region has exactly one open cell left. Placing here clears the rest of row 8, column A, and its region.
Now, place a marker at E7. Region has exactly one open cell left. Placing here clears the rest of row 7, column E, and its region.
Finally, place a marker at G4. Region has exactly one open cell left.

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