Demonic Spiders

Dragondawn
* The plants are tougher than the other two enemies and can make the entire group go berserk.
* The monster balls, unless you buy anti-magic-items could crush the group in a few turns.
* The starers and ogres. Both are very tough, while the former tends to confuse the group the latter could annihilate the group with basic skills.
* The demons later in the same cave are basically monster balls and ogres put in one, but thankfully the least durable of the five foes encountered.
The Legend of Devils
* The snakes. They're tough and they can poison the hero/es. That said, after the first dungeons they're not too hard to deal with.
* The knights (Dungeon 6). They can survive many blows and have a devastating skill for the entire group. Thankfully, they only appear alone there. Later, there are similar enemies (Big Gargoyles; Mummies) and unlike those they don't always appear alone, which is why the eighth castle and the pyramid are quite possibly the hardest dungeons in the game.