How to solve this 8×8 logic puzzle

8×8 · severe · 26 steps

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This 8×8 board rates as severe (difficulty score 1304). 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 C7, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, 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.
Then, 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.
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, 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.
From there, 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.
Following that, every remaining candidate in this region touches E3 and G3 and E4 and G4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
First, every remaining candidate in this region touches C3, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, every remaining candidate in this region touches B5 and C5, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
Then, marking A1 would force D5 (the last open cell in region 5), then B6 (the last open cell in region 7), and then region 8 would be left with no open cell for its marker. So A1 can't be the marker there.
After that, if B1 were the marker, it would force C6 (the last open cell in region 7), then A5 (the last open cell in region 5), and region 8 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 B6 (the last open cell in region 7), then D5 (the last open cell in region 5), and region 3 would be left with no open cell for its marker.
From there, marking D1 would force A5 (the last open cell in region 5), then C4 (the last open cell in region 3), and then row 6 would be left with no open cell for its marker. So D1 can't be the marker there.
Following that, marking G1 would force E8 (the last open cell in column E), then A7 (the last open cell in region 8), then D5 (the last open cell in region 5), and then row 6 would be left with no open cell for its marker. So G1 can't be the marker there.
First, if H1 were the marker, it would force A7 (the last open cell in row 7), then D5 (the last open cell in region 5), and row 6 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. Placing here also clears its 1 touching neighbour.
Then, if C2 were the marker, it would force B6 (the last open cell in region 7), then D5 (the last open cell in region 5), then A8 (the last open cell in region 8), and column G would end up with no open cell for its marker, which can't happen. So it can't go there.
After that, every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
Now, marking H2 would force G8 (the last open cell in region 6), then A7 (the last open cell in region 8), then D5 (the last open cell in region 5), and then row 6 would be left with no open cell for its marker. So H2 can't be the marker there.
From there, 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.
Following that, place a marker at F4. Region has exactly one open cell left. Placing here clears the rest of row 4, column F, and its region.
First, place a marker at D3. Region has exactly one open cell left. Placing here clears the rest of row 3, column D, and its region.
Next, place a marker at A5. Region has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region. Placing here also clears its 1 touching neighbour.
Then, place a marker at C6. Region has exactly one open cell left.
After that, place a marker at B8. Region has exactly one open cell left. Placing here clears the rest of row 8, column B, and its region.
Finally, place a marker at H7. Region has exactly one open cell left.

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