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Classic sudoku N°6

During this first tournament, I spent a huge time to try to solve the classic sudoku N°6 logically (or at least find a contradiction that allows me to remove some candidates and lead to the solution). At the end, death in the soul, I decided to guess to find the solution...
(I also had to guess on classic sudoku N°5).

Can anybody explain me how to solve logically this sudoku?


This is set of techniques for solving No.6 - 78 points:
Forcing X-chain
twice Forcing chain
twice X-Wing
twice Forcing chain
It is not logic solving, it is comedy.
Classic No.2 28 points - XYZ-Wing, Turbotfish
Classic No.4 46 points - Turbotfish, Swordfish
Classic No.5 55 points - X-Wing, XY-Wing

So, in my opinion, only the number 1 and 5 corresponds to the quality and scoring. While on the other hand, for the same 55 points, beautiful and very brisk puzzle "Stars".
Overall, this really does not seem for a good start of the second year of GP. I look forward to Serbian round.

I decided to guess number in Classic No 4 & 5 & 6 and I was lucky because my first guess in these classics was true.

While we told all authors to provide more classics this year, each country has had their own approach to delivering these puzzles and certainly on how hard to make them. After the full set of contests, we will welcome solver feedback on what were good/bad design choices that we can enforce more strongly for next year.

So, solvers should be aware that our points are primarily based on how long it would take a solver to complete the puzzles in a standard competition approach which can (unfortunately) involve guessing. For example, the ending of classic #2 is definitely tricky (I used coloring on a digit), but there are only about 12 cells involved at that point so it is impossible to rate that puzzle as much higher value as solvers will be very close to done much much sooner than on the other harder puzzles.