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 1206). Solving it from scratch takes 32 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

First, marking A1 would force E8 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
Next, 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.
Then, every remaining candidate in this region touches E2, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, if B1 were the marker, it would force E8 (the last open cell in region 8), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
Now, that one can't be the marker: at C1 it would force D3 (the last open cell in region 2), and region 5 would be left with no open cell for its marker.
From there, every open cell left in this region sits in column B, so its marker has to land there; that clears every other open cell in the column.
Following that, every open cell left in this region sits in column A, so its marker has to land there; that clears every other open cell in the column.
First, every remaining candidate in this region touches A2 and A3 and C3, 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 A4. Region has exactly one open cell left. Placing here clears the rest of row 4, column A, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at B2. Region has exactly one open cell left. Placing here clears the rest of row 2, column B, and its region.
After that, marking D1 would force E8 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So D1 can't be the marker there.
Now, that one can't be the marker: place it at F1, and region 3 would be left with no open cell for its marker.
From there, marking G1 would force D3 (the last open cell in region 2), then E8 (the last open cell in region 8), and then region 6 would be left with no open cell for its marker. So G1 can't be the marker there.
Following that, if H1 were the marker, it would force D3 (the last open cell in region 2), then E8 (the last open cell in region 8), and region 6 would end up with no open cell for its marker, which can't happen. So it can't go there.
First, 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.
Next, every open cell left in this region sits in column F, so its marker has to land there; that clears every other open cell in the column.
Then, every open cell in column H 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.
After that, place a marker at G8. Column 7 has exactly one open cell left. Placing here clears the rest of row 8, column G, and its region. Placing here also clears its 1 touching neighbour.
Now, place a marker at D7. Region has exactly one open cell left. Placing here clears the rest of row 7, column D, and its region. Placing here also clears its 1 touching neighbour.
From there, place a marker at C5. Region has exactly one open cell left. Placing here clears the rest of row 5, column C, and its region.
Following that, 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.
Finally, place a marker at H6. Region has exactly one open cell left.

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