How to solve this 6×6 logic puzzle

6×6 · sharp · 13 steps

This 6×6 board rates as sharp (difficulty score 1016). Solving it from scratch takes 20 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 row 5, column 3 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, marking row 1, column 2 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Then, marking row 1, column 3 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
After that, marking row 1, column 4 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Now, marking row 1, column 5 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
From there, every open cell left in this region sits in column 1, so its marker has to land there — clearing every other open cell in the column.
Following that, every open cell left in this region sits in column 2, so its marker has to land there — clearing every other open cell in the column.
First, place a marker at row 6, column 3. Region has exactly one open cell left. Placing here clears the rest of row 6, column 3, and its region. Placing here also clears its 1 touching neighbour.
Next, place a marker at row 4, column 4. Region has exactly one open cell left. Placing here clears the rest of row 4, column 4, and its region. Placing here also clears its 2 touching neighbours.
Then, place a marker at row 5, column 6. Region has exactly one open cell left. Placing here clears the rest of row 5, column 6, and its region.
After that, place a marker at row 2, column 5. Region has exactly one open cell left. Placing here clears the rest of row 2, column 5, and its region.
Now, place a marker at row 3, column 2. Region has exactly one open cell left. Placing here clears the rest of row 3, column 2, and its region.
Finally, place a marker at row 1, column 1. Region has exactly one open cell left.

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