Master Sudoku puzzles are rated 7.0–9.9 by Sudoku Explainer, the range where forcing chains, Nishio and dynamic nets become necessary. There is no quick win: a master grid is solved one carefully verified elimination at a time, often over an hour or more.
These puzzles are meant for solvers who enjoy the reasoning itself, not just the finished grid. Your board is saved after every move, so you can leave a master puzzle overnight and return exactly where you stopped. Want the biggest version of this challenge? Play master 16x16 Sudoku.