How to solve this 6×6 logic puzzle

6×6 · sharp · 15 steps

This 6×6 board rates as sharp (difficulty score 1008). Solving it from scratch takes 20 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, place a marker at row 4, column 6. Region has exactly one open cell left. Placing here clears the rest of row 4, column 6, and its region. Placing here also clears its 2 touching neighbours.
Next, every open cell left in this region sits in column 4, so its marker has to land there — clearing every other open cell in the column.
Then, every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row — clearing every other open cell in the region.
After that, together, two regions have open cells only in rows 5 and 6 — 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.
Now, every remaining candidate in this region touches row 3, column 3 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
From there, marking row 1, column 1 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Following that, 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.
First, together, two regions have open cells only in columns 3 and 5 — 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.
Next, every remaining candidate in this region touches row 2, column 4 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Then, place a marker at row 3, column 4. Region has exactly one open cell left. Placing here clears the rest of row 3, column 4, and its region.
After that, place a marker at row 2, column 1. Region has exactly one open cell left. Placing here clears the rest of row 2, column 1, and its region.
Now, 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.
From there, place a marker at row 6, column 5. Region has exactly one open cell left. Placing here clears the rest of row 6, column 5, and its region.
Following that, place a marker at row 1, column 3. Region has exactly one open cell left.
Finally, place a marker at row 5, column 2. Region has exactly one open cell left.

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