Notes on this puzzle
Sudoku #5312 is a hard grid with 26 givens, which leaves 55 cells waiting for a digit. Played purely by logic, it takes 59 forced steps from the first placement to the last.
Most cells fall to the hidden single move: 48 of the 59 steps. Single candidate accounts for 7 more. The toughest moment needs box–line reduction, and that single step is what sets the level.
Among the givens, 2 is the most common digit (5 times) and 6 the rarest (1). The emptiest box is the top-left one, with 2 givens; the middle-left box is the most crowded, with 4.
Where to beginLook at row 5, 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.