How to solve this 8×8 logic puzzle

8×8 · severe · 20 steps

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This 8×8 board rates as severe (difficulty score 1186). Solving it from scratch takes 30 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, Region 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 row 4, so its marker has to land there; that clears every other open cell in the row.
Next, every remaining candidate in this region touches B3 and B5, 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 B7, 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 F2 (the last open cell in region 2), then H3 (the last open cell in row 3), then G8 (the last open cell in region 6), and the chain continues, and then row 6 would be left with no open cell for its marker. So A1 can't be the marker there.
Now, if B1 were the marker, it would force F2 (the last open cell in region 2), then H3 (the last open cell in row 3), then G8 (the last open cell in region 6), and row 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
From there, that one can't be the marker: at C1 it would force F2 (the last open cell in region 2), then H3 (the last open cell in row 3), then G8 (the last open cell in region 6), and the chain continues, and column D would be left with no open cell for its marker.
Following that, if E1 were the marker, region 2 would end up with no open cell for its marker, which can't happen. So it can't go there.
First, that one can't be the marker: at F1 it would force E5 (the last open cell in region 4), and column D would be left with no open cell for its marker.
Next, marking G1 would force H8 (the last open cell in column H), and then column F would be left with no open cell for its marker. So G1 can't be the marker there.
Then, if H1 were the marker, it would force G8 (the last open cell in column G), then F2 (the last open cell in column F), then B4 (the last open cell in column B), and row 3 would end up with no open cell for its marker, which can't happen. So it can't go there.
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. Placing here also clears its 1 touching neighbour.
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, 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. Placing here also clears its 1 touching neighbour.
Following that, place a marker at H3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
First, place a marker at G8. Region has exactly one open cell left. Placing here clears the rest of row 8, column G, and its region.
Next, every remaining candidate in this region touches B6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Then, place a marker at B4. Column 2 has exactly one open cell left. Placing here clears the rest of row 4, column B, and its region. Placing here also clears its 2 touching neighbours.
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.
Now, place a marker at C7. Region has exactly one open cell left. Placing here clears the rest of row 7, column C, 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. ∎