Cosmo's Cosmic Adventure 2 - Levels 5 and 6
Level 5 is fairly nonlinear, which makes it interesting. It's otherwise pretty straightforward though, so nothing much to say about it.
Level 6, on the other hand, has quite a bit going on. It's probably the hardest level I've seen so far (it's the only one that's remotely difficult in the first place), primarily because there are no health items throughout the stage. You get a bunch at the beginning and the end, but nothing in between.
Also, I'm pretty sure those areas where there is no floor are supposed to kill you if you fall down there, but they don't. Must be a glitch or something, seeing as any enemies or items that fall in will indeed disappear.
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