How to solve this 8×8 logic puzzle

8×8 · severe · 23 steps

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This 8×8 board rates as severe (difficulty score 1308). Solving it from scratch takes 32 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 remaining candidate in this region touches B3, 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 G2 (the last open cell in region 2), then H4 (the last open cell in region 5), then C3 (the last open cell in region 3), and the chain continues, and then region 7 would be left with no open cell for its marker. So A1 can't be the marker there.
Then, every remaining candidate in this region touches C2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, that one can't be the marker: at C1 it would force G2 (the last open cell in region 2), then H4 (the last open cell in region 5), then A3 (the last open cell in region 3), and the chain continues, and region 7 would be left with no open cell for its marker.
Now, marking D1 would force G2 (the last open cell in region 2), then H4 (the last open cell in region 5), then C5 (the last open cell in region 4), and the chain continues, and then region 7 would be left with no open cell for its marker. So D1 can't be the marker there.
From there, that one can't be the marker: at A2 it would force D3 (the last open cell in region 1), then C5 (the last open cell in region 4), and region 7 would be left with no open cell for its marker.
Following that, marking G1 would force C5 (the last open cell in column C), then B2 (the last open cell in column B), and then column D would be left with no open cell for its marker. So G1 can't be the marker there.
First, marking B2 would force D5 (the last open cell in region 4), and then column C would be left with no open cell for its marker. So B2 can't be the marker there.
Next, if E1 were the marker, it would force D3 (the last open cell in region 1), then C5 (the last open cell in region 4), then A4 (the last open cell in region 3), and region 7 would end up with no open cell for its marker, which can't happen. So it can't go there.
Then, that one can't be the marker: at D2 it would force C5 (the last open cell in region 4), and region 7 would be left with no open cell for its marker.
After that, that one can't be the marker: at F1 it would force D3 (the last open cell in region 1), then C5 (the last open cell in region 4), then A4 (the last open cell in region 3), and region 7 would be left with no open cell for its marker.
Now, every remaining candidate in this region touches H2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
From there, if H1 were the marker, it would force D3 (the last open cell in region 1), then C5 (the last open cell in region 4), then A4 (the last open cell in region 3), and region 7 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 G2. Region has exactly one open cell left. Placing here clears the rest of row 2, column G, and its region. Placing here also clears its 2 touching neighbours.
First, place a marker at H4. Region has exactly one open cell left. Placing here clears the rest of row 4, column H, and its region.
Next, place a marker at B1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column B, and its region.
Then, every open cell left in this region sits in row 3, so its marker has to land there; that clears every other open cell in the row.
After that, 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.
Now, place a marker at F6. Region has exactly one open cell left. Placing here clears the rest of row 6, column F, and its region. Placing here also clears its 1 touching neighbour.
From there, place a marker at E8. Column 5 has exactly one open cell left. Placing here clears the rest of row 8, column E, and its region.
Following that, place a marker at A7. Region has exactly one open cell left. Placing here clears the rest of row 7, column A, and its region.
First, place a marker at C3. Region has exactly one open cell left. Placing here clears the rest of row 3, column C, and its region.
Finally, place a marker at D5. Region has exactly one open cell left.

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