How to solve this 8×8 logic puzzle

8×8 · severe · 27 steps

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This 8×8 board rates as severe (difficulty score 1463). Solving it from scratch takes 35 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, 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.
Next, marking A1 would force D5 (the last open cell in region 4), then B6 (the last open cell in region 7), then C8 (the last open cell in region 8), and the chain continues, and then row 2 would be left with no open cell for its marker. So A1 can't be the marker there.
Then, if B1 were the marker, it would force C6 (the last open cell in region 7), and region 4 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 C1 it would force B8 (the last open cell in region 8), and region 7 would be left with no open cell for its marker.
Now, marking D1 would immediately leave region 7 with no open cell for its marker, so it can't be the marker there.
From there, that one can't be the marker: place it at F1, and column G would be left with no open cell for its marker.
Following that, marking G1 would force E2 (the last open cell in column E), then A3 (the last open cell in row 3), then H4 (the last open cell in row 4), and the chain continues, and then row 8 would be left with no open cell for its marker. So G1 can't be the marker there.
First, that one can't be the marker: at A2 it would force D5 (the last open cell in region 4), then E1 (the last open cell in region 2), then F3 (the last open cell in region 5), and region 3 would be left with no open cell for its marker.
Next, marking B2 would force E1 (the last open cell in region 2), then G3 (the last open cell in row 3), and then row 4 would be left with no open cell for its marker. So B2 can't be the marker there.
Then, if C2 were the marker, it would force E1 (the last open cell in region 2), then B8 (the last open cell in region 8), and column A 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 D2 it would force H1 (the last open cell in row 1), and region 7 would be left with no open cell for its marker.
Now, marking E2 would force H1 (the last open cell in row 1), then D5 (the last open cell in region 4), and then region 5 would be left with no open cell for its marker. So E2 can't be the marker there.
From there, that one can't be the marker: at G2 it would force E1 (the last open cell in row 1), then A3 (the last open cell in row 3), then H4 (the last open cell in row 4), and the chain continues, and row 8 would be left with no open cell for its marker.
Following that, if A3 were the marker, it would force D5 (the last open cell in region 4), and row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
First, if H1 were the marker, it would force F2 (the last open cell in row 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, place a marker at E1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column E, and its region.
Then, place a marker at H2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column H, and its region.
After that, together, two regions have open cells only in rows 7 and 8; with one marker each, those two markers have to fill exactly those two rows, so every other region's open cells there can be cleared.
Now, every open cell left in this region sits in row 6, so its marker has to land there; that clears every other open cell in the row.
From there, every open cell in column A 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, together, two regions have open cells only in columns B and C; 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.
First, 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. Placing here also clears its 1 touching neighbour.
Next, place a marker at F3. Region has exactly one open cell left. Placing here clears the rest of row 3, column F, and its region.
Then, place a marker at A4. Region has exactly one open cell left.
After that, 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.
Now, place a marker at C8. Region has exactly one open cell left. Placing here clears the rest of row 8, column C, and its region.
Finally, place a marker at G7. Region has exactly one open cell left.

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