Notes on this puzzle
Sudoku #5408 is an expert grid with 30 givens, which leaves 51 cells waiting for a digit. Played purely by logic, it takes 56 forced steps from the first placement to the last.
Most cells fall to the hidden single move: 48 of the 56 steps. Single candidate accounts for 3 more. The toughest moment needs XY-Wing, and that single step is what sets the level.
Among the givens, 6 is the most common digit (5 times) and 1 the rarest (2). The emptiest box is the top-left one, with 1 given; the top-middle box is the most crowded, with 5.
Where to beginLook at row 2, column 1 first: a hidden single is available there from the very start. Expect a stage where only a pattern across several lines moves the grid on; full notes make it visible.