Free Sudoku with hints that teach the technique

Difficulty is the main tier of the progression. Rookie: the basic techniques. Apprentice: intermediate, with pair and reduction techniques. Enthusiast: more advanced techniques involving triples. Specialist: builds on the basics and requires the named Specialist technique for the sublevel. Technician: needs at least two Specialist breakthroughs in the same puzzle. It has no sublevels. A higher track generally means more advanced logic.