The level in Isu's avatar looks almost exactly like 6-9 in the SNES version. One of the 6-9s, at least.
I remember the colors, so it must have been 6-7 extra that kills me. Also, as a child, 6-4 (1st set) gave me a LOT of problems.
Ya, 6-8 took about 5 minutes for me, but I can definitely see how it'd be hard, especially given the layout and the number of solutions that are very close to working but are nothing like the correct one.
Anyways, back to the Hubris set. I've cleared all but 36 and 40 (Well, 37 required a move during a move for me, so I think I solved that one in an unintended way). Seriously, 40 is kind of frustrating -- I've logged about 40 minutes on it now and nothing. The number of "one off" solutions I've found... well, that and about 70% of the leads I've tried getting stopped by accidental clears. Really, it's so easy to make clears you don't want to with that layout.
Of course, I do applaud a stage that stumps me for so long -- I think maybe 3 puzzles total in the original Tetris Attack took me longer than 40 minutes of solve time. And 36 is kind of getting up there, also. Seriously, for two puzzles that seem like there's not a lot to try, they're proving quite a difficult task for me. I'm overall quite impressed with Nuntar's set of puzzles -- a lot of them are the quality of the puzzles in the actual game.
Anyways, I've decided to start my own set of 60 puzzles... I'm only using 3 of the 5 puzzles I uploaded as a demo, but I don't know how long they'll take.