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.
Demonic Spiders: B. Lance has an erratic bouncing pattern, a capsule that lets them go twice, a fairly high encounter rate, and they're powerful even if you're at a good level for other enemies in the area.
In later dungeons, B. Dragons and B. Zeppelins pick up the slack.
I Am Not Shazam: The robot that follows you and NPCs around is a Metal Walker. Enemy robots and those controlled by Metal Masters are Metal Busters. There's a difference.
B. Dragon. He does a fair amount of damage even if you grinded... and his amount of HP is massive. Even if you do 77 damage with each hit, he will likely kill you before you kill him, especially if he or his minions get good capsules.
That One Level: Bronze Rocks, as it's a Marathon Level with powerful enemies and an even more powerful boss at the end that can only be accessed through one, long route.
Neo City, the final level, is much the same—and its enemies are even harder, with a very long path to get back to the final boss. Even worse is that some areas are dead ends.