Expert Sudoku
Expert Sudoku is the ultimate test of logic and deduction. With just 22 to 27 givens, these puzzles require every advanced technique in your arsenal. Only seasoned solvers should attempt expert level.
At expert level, you will need techniques like X-wing — when a candidate appears in exactly two cells in each of two rows (or columns), forming a rectangle, you can eliminate that candidate from other cells in those columns (or rows). Pointing pairs and box/line reduction become routine.
Chain logic may also be required: following a sequence of implications from one candidate placement to see if it leads to a contradiction. Expert puzzles demand focus and persistence, but the satisfaction of completing one is unmatched.