Hardcore
Three 8×8 and five 9×9 boards with 6 to 9 threads — the classic levels our solver rated hardest.
Each level was checked by the solver: it has exactly one solution, and reaching it takes more than forced moves. Expect several places where you have to hold two or three options in your head and see which one breaks the board further on. The set gets harder towards the end.
Work in regions instead of tracing one thread across the whole board: settle the corners, then the edges, then the middle. Big boards explains how, and using the single solution helps when two routes look equally good.