Most of the diagonal sudokus at Menneske's site aren't overly hard (for me). The hardest available setting is usually solvable with X-wings (which are fun when they involve diagonals!).
This one seems notably harder than most. I used color chains (forcing chains on a single number, but beyond simple coloring) to crack it.
By the way, diagonal sudokus can have 3-edge forcing chains, which can't occur in normal sudoku. "Delta-wings" seems like a reasonable name for these shortest non-trivial forcing chains.
P.S.- Just discoved why it was so hard. I accidentally omitted a clue (the initial 7 at r7c2). Still solvable, just hard