Puzzle designer Yuu Asaka's creations are ranked in categories 8 (Complex), 9 (Very Difficult) and 10 (Insane) on PuzzleScore, and it's easy to see why. Asaka has turned jigsaw puzzle design on its head: Here the pieces don't need to interlock, but they must all fit into the included tray. Some of the puzzles have five corner pieces; others have more holes than circles to fill them. Some examples:
Oleo Puzzle 10
Wave Puzzle 5
Ice Puzzle 9
Wave Puzzle 7
PuzzleScore says some of these may take up to three days to solve.
While I admire Asaka's unconventional thinking, any one of these puzzles would drive me nuts.