These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
That One Boss: Wood Man. His Ice Circle moves much slower than the Leaf Shield (enough to ruin any attempt to jump over it), and the four other snowflakes (leaves) move insanely fast. It's practically impossible to defeat him without the Delay Flame, so don't bother trying.
That One Level: Flash Man's stage: tons of enemies placed in a way to suggest the author was trying to piss you off, lots of bottomless pits, and an annoying boss (who is probably hell without the Wind Slicer) to top it all off.