You can still make levels that are both fun and "a pain to beat."
Anyway, I did see this new level. And here are my thoughts.
The level feels stretched out and needlessly big; I think it could be smaller or trimmed of the filler, such as jumping up nine platforms of unavoidable flames (which in itself is a bad design choice) or slowly moving through a piranha plant shaft 10 pipes long.
The abundance and placement of Coins is questionable. Coins in Mario games are given as rewards for jumping a certain way or performing a certain task. Even NSMB2, which seems to shower coins at any given turn, still gave them in exchange for the player doing something, which made collecting them feel awarding. In your levels Mario mostly walks or falls into 100 coins at once, so there's even less effort needed to collect them than what was needed in NSMB2.
You also made it way too easy for the player to reach of the top of the Goal Flag.
In the moving water shaft, you didn't offer any visible initiative or incentive for the player to go down the Cheep-Cheep path, so the player will more likely take the path that is closer, moves faster, and has no visible hazards. And it was obvious there was no reward for going down the Cheep-Cheep path anyway, which made the invisible coin blocks redundant since the player is not inclined to go back. This makes the Cheep-Cheep path completely pointless.
Back to the unavoidable flames: I think levels that make the player dependent on power-ups (like your Level 4) are bad because power-ups are not replenishable and will often screw the player over. In normal 2D Mario games they serve only to make progressing easier and are seldom necessary to beat any level. Forcing Mario to shrink near the end is completely counter-intuitive and, given that you had placed a Mushroom right before that, makes no sense. If the player can complete the level as small, large, and all other forms of Mario from start to finish (and able to completely avoid all enemies and power-ups at the same time), then I consider it a well-designed level.