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: With sufficiently powerful spells, a group of spellcasters will tear your party apart in a single turn.
Goddamn Bats: Spellcasters. Any spellcasters. Especially if they attack in groups. Except for some Attribute spells, all spells attack the entire party one at a time, and if a ton of them decide to start spamming spells, it could be a long time before you can act again.
Similarly, any enemies that can inflict status ailments (usually Sleep.) It's so much fun when your entire party is put to sleep and you're only left with your spirit (who's immune) trying to cast spells or just chip away at them with your puny physical attacks to get rid of them.
That One Level: The Ice Mine. Absolutely massive and labyrinthine, full of staircases to take you to different floors, which makes it hard to keep track of where you are even with the in-game map, and chock full of tough enemy encounters that usually group up in mobs of 5 or more (and lots of them are spellcasters.) Rooks also has to solo the beginning of the dungeon until he can track down Axs.