Master Sudoku is for solvers who enjoy sustained logical deduction. Progress often comes from combining multiple candidate eliminations before placing a digit.
Regular advanced solvers seeking a demanding session.
Recheck every candidate after each placement. Look for pairs shared across units, then revisit cells that have lost a candidate. A chain of small eliminations can open the next definite move.
On a keyboard, use the arrow keys to move between cells and N to toggle notes. Read the beginner guide or try a worked naked-single example.