Notes on this puzzle
Sudoku #5404 is an expert grid with 26 givens, which leaves 55 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: 53 of the 56 steps. Single candidate accounts for 2 more. The toughest moment needs XY-Wing, and that single step is what sets the level.
Among the givens, 5 is the most common digit (4 times) and 3 the rarest (2). The emptiest box is the top-right one, with 2 givens; the top-middle box is the most crowded, with 4.
Where to beginLook at row 6, column 3 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.