Notes on this puzzle
Sudoku #5304 is a hard grid with 31 givens, which leaves 50 cells waiting for a digit. Played purely by logic, it takes 55 forced steps from the first placement to the last.
Most cells fall to the hidden single move: 48 of the 55 steps. Naked pair accounts for 2 more. The toughest moment needs box–line reduction, and that single step is what sets the level.
Among the givens, 6 is the most common digit (5 times) and 2 the rarest (2). The emptiest box is the top-middle one, with 3 givens; the top-left box is the most crowded, with 5.
Where to beginLook at row 1, column 5 first: a hidden single is available there from the very start. Pencil in candidates for one box at a time and watch for two cells that share the same pair.