DC Examiner's Diabolical Sudoku Classic Puzzle

Our local newspaper, the DC Examiner, recently began running two Sudoku puzzles each day-- Sudoku Classic and Sudoku Pacific. Generally speaking, the Monday puzzles are easy and the difficulty level progresses through the week. By the time you get to the weekend edition, you are looking at maximum skill level puzzles. I know my Sudoku solving techniques are incomplete, so I've been drilling myself with these puzzles to try to force myself to improve.

I could be mistaken, but I think the folks in charge of the Examiner's "Games!" page decided to throw us a curve ball this weekend by featuring a "diabolical" puzzle for the Sudoku Classic (shown below). It's marked with 6 out of 6 stars, and about two-thirds of the way through working the puzzle, you reach a point where it becomes impossible to eliminate any more possibilities.



You don't see many diabolical Sudoku puzzles in mainstream newspapers these days. (It tends to upset the so-called Sudoku purists who insist that all puzzles must be solved by logic alone and should never involve any guesswork.) There is a technique called "Ariadne's thread", which I have not mastered, that might address such situations-- but let's be honest here, it amounts to picking a cell with only two possible values, choosing one of those values and solving forwards from there until you either encounter a conflict or solve the puzzle. If you do find a conflict, you erase your way "backwards" to the point before you picked that value, and then you select the other option and progress forwards. Using a photo copier or different colored pencils makes this process much easier.