How to solve this 8×8 logic puzzle

8×8 · severe · 22 steps

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This 8×8 board rates as severe (difficulty score 1270). Solving it from scratch takes 32 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, 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.
Next, every remaining candidate in this region touches F3 and H3 and F4 and H4, 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 E7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, marking A1 would force H2 (the last open cell in region 2), then G4 (the last open cell in region 5), then B3 (the last open cell in row 3), and then column C would be left with no open cell for its marker. So A1 can't be the marker there.
Now, marking D1 would force H2 (the last open cell in region 2), then G4 (the last open cell in region 5), then A3 (the last open cell in row 3), and then column B would be left with no open cell for its marker. So D1 can't be the marker there.
From there, if H1 were the marker, it would force F7 (the last open cell in region 7), then D8 (the last open cell in region 8), then E5 (the last open cell in region 6), and the chain continues, and row 2 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 A2 it would force G4 (the last open cell in region 5), then C3 (the last open cell in column C), then E6 (the last open cell in row 6), and row 5 would be left with no open cell for its marker.
First, if E1 were the marker, it would force G4 (the last open cell in region 5), then A5 (the last open cell in column A), then B2 (the last open cell in column B), and column C would end up with no open cell for its marker, which can't happen. So it can't go there.
Next, marking B2 would immediately leave column C with no open cell for its marker, so it can't be the marker there.
Then, if C2 were the marker, it would force E3 (the last open cell in region 1), then F1 (the last open cell in region 2), then G4 (the last open cell in region 5), and the chain continues, and column B would end up with no open cell for its marker, which can't happen. So it can't go there.
After that, that one can't be the marker: at F1 it would force D2 (the last open cell in row 2), then E8 (the last open cell in region 8), then H7 (the last open cell in region 7), and region 6 would be left with no open cell for its marker.
Now, place a marker at H2. Region has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region. Placing here also clears its 1 touching neighbour.
From there, place a marker at G4. Region has exactly one open cell left. Placing here clears the rest of row 4, column G, and its region. Placing here also clears its 1 touching neighbour.
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.
First, every remaining candidate in this region touches E6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, place a marker at F7. Region has exactly one open cell left. Placing here clears the rest of row 7, column F, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at D8. Region has exactly one open cell left. Placing here clears the rest of row 8, column D, and its region.
After that, place a marker at E5. Region has exactly one open cell left. Placing here clears the rest of row 5, column E, and its region.
Now, that one can't be the marker: place it at B3, and row 6 would be left with no open cell for its marker.
From there, place a marker at A3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column A, and its region.
Following that, 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.
Finally, place a marker at B1. Region has exactly one open cell left.

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