Notes on this puzzle
Sudoku #5308 is a hard grid with 28 givens, which leaves 53 cells waiting for a digit. Played purely by logic, it takes 54 forced steps from the first placement to the last.
Most cells fall to the hidden single move: 51 of the 54 steps. Single candidate accounts for 2 more. The toughest moment needs box–line reduction, and that single step is what sets the level.
Among the givens, 7 is the most common digit (5 times) and 3 the rarest (0). 1 of the nine digits do not appear among the givens at all; every one of their positions has to be deduced. The emptiest box is the central one, with 2 givens; the middle-left box is the most crowded, with 4.
Where to beginLook at row 5, column 1 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.