I think Level 5 was largely my favorite run of levels, mostly because it took pretty cool use of bookshelves hiding crawling hands coming at you, and the bottomless pits in the containment tubes were easier to avoid and enemies typically didn't spawn to push you in them. The sewers and lab get a difficulty spike, but it's still a test of patience overall.

The portal level sucked. The amount of health just isn't enough for what is memorization of patterns with some otherwise unreactable segments in the chaos.

The seventh string of levels was also fine! The void's aesthetic is cool, and I like how the eventual escape is basically running away from Brawl's Tabuu with some glowing orbs. The difficulty of the second part, removing the girl from the crystal, is pretty punishing, though.

The final section of levels is where it really gets grating in terms of options and movement, though. The patterns of the elevator drop are fine, but the final boss is heavy reaction time + patterns with a high difficulty curve. It's not impossible and the difficulty is a challenge, but it really pushes how little your movement and combat options are, where the last section of the final boss requires nearly perfect execution with movement I already wasn't a fan of.

And that makes 100% completion today.
Scrap Brain isn't as bad as Labyrinth Zone but it's close. Pro tip: Go really fucking slow. I know, it sucks, but that is how you beat it. Every single trap feels like it's asking you to rush past it so you miss signposting and get zapped or murdered, and there are blocks that you have to wait to appear. Enemy design isn't nearly as bad as Labyrinth Zone, though, just traps are worse. That being said, having to play through it so slow to function is distressing, if not the antithesis to what the game seemingly wants?
The final boss? No rings. I thankfully got through it unscathed, finding the electric balls easy to react to, and positioning myself so I always had an out. Never got hit once, although asking for the player to never get hit is still asking too much. Made the boss kind of a joke for me, though. If you're impatient, you're dead.

The last three achievements on RetroAchievements were invented by people who felt like abusing the player, at least in my book. Completing Marble Zone in less than 90 seconds is a neat challenge, and actually not that hard to do. The underachiever and 200 rings achievements? Ridiculous. The underachiever asks you to wait 5 minutes in every stage of Green Hill, and have 0 rings when you get there, AND have hit no enemies. Green Hill is already a slightly unfair stage, but now it's completely jacked up, and waiting is painful without speed on emulation. But the 200 rings might be the worst one simply because most stages are in the 210-230 range if they're even close, which means backtracking through each path of a level. I walked out of a stage with only 198 because some were impossible to get in one go, and said fuck it. Time for cheats. I'm not sorry.

24 of 24 achievements.