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 1170). Solving it from scratch takes 31 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 B7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, every open cell left in this region sits in column C, so its marker has to land there; that clears every other open cell in the column.
Then, 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.
After that, 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.
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, every remaining candidate in this region touches F3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Following that, every remaining candidate in this region touches D7 and B8 and D8, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
First, marking A1 would force B6 (the last open cell in region 7), then D4 (the last open cell in region 5), then E2 (the last open cell in region 4), and then region 2 would be left with no open cell for its marker. So A1 can't be the marker there.
Next, that one can't be the marker: at F1 it would force B6 (the last open cell in column B), then D4 (the last open cell in region 5), and column E would be left with no open cell for its marker.
Then, every remaining candidate in this region touches G3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, marking G1 would force F4 (the last open cell in region 2), and then row 5 would be left with no open cell for its marker. So G1 can't be the marker there.
Now, if H1 were the marker, it would force F5 (the last open cell in row 5), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
From there, 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.
Following that, marking E2 would force F4 (the last open cell in region 2), then D6 (the last open cell in region 5), then C8 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So E2 can't be the marker there.
First, every remaining candidate in this region touches D4 and F4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Next, place a marker at D6. Region has exactly one open cell left. Placing here clears the rest of row 6, column D, and its region. Placing here also clears its 1 touching neighbour.
Then, 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.
After that, place a marker at F5. Region has exactly one open cell left. Placing here clears the rest of row 5, column F, and its region. Placing here also clears its 2 touching neighbours.
Now, 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 1 touching neighbour.
From there, place a marker at E3. Region has exactly one open cell left.
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.
Finally, place a marker at H4. Region has exactly one open cell left.

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