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 1146). Solving it from scratch takes 27 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region 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 A6, 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 C7 (the last open cell in region 7), then B5 (the last open cell in region 6), then D4 (the last open cell in region 4), and the chain continues, and then region 8 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 A5 (the last open cell in region 6), then C7 (the last open cell in region 7), then D4 (the last open cell in region 4), and the chain continues, and region 8 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: place it at C1, and column H would be left with no open cell for its marker.
Now, together, two regions have open cells only in columns A and B; 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 C7. Region has exactly one open cell left. Placing here clears the rest of row 7, column C, and its region. Placing here also clears its 3 touching neighbours.
Following that, place a marker at E8. Region has exactly one open cell left. Placing here clears the rest of row 8, column E, and its region.
First, every open cell left in this region sits in column D, so its marker has to land there; that clears every other open cell in the column.
Next, every open cell left in this region sits in row 1, so its marker has to land there; that clears every other open cell in the row.
Then, 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.
After that, place a marker at D4. Region has exactly one open cell left. Placing here clears the rest of row 4, column D, and its region.
Now, every remaining candidate in this region touches F2 and G2, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
From there, marking H2 would immediately leave column G with no open cell for its marker, so it can't be the marker there.
Following that, place a marker at A2. Row 2 has exactly one open cell left. Placing here clears the rest of row 2, column A, and its region.
First, place a marker at B5. Region has exactly one open cell left.
Next, marking F3 would immediately leave column H with no open cell for its marker, so it can't be the marker there.
Then, if G3 were the marker, row 6 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 H3. Row 3 has exactly one open cell left. Placing here clears the rest of row 3, column H, and its region.
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.
Finally, place a marker at G1. Region has exactly one open cell left.

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