That One Boss

4 vs. Satan:
* The two Final Bosses, the first is a Marathon Boss, while the second is a Glass Cannon, who fights the main character 1-one-1, and if you aren't lucky enough, could trash the hero. Later versions change him with a more balanced boss... at the cost of giving him more HP.
The 3 Kingdoms:
* Any boss starting with Bertram. They have vast attack and/or magic power and can tank a lot.
The Stone of Power:
* OR: The minotaur and the sphinx if you don't opt to poison them. The first because he has lots of HP for how early he appears, the second because he can debuff the heroes while boosting himself.
* The second-to-last boss, as one of them spams a debuff spell, while all three fire powerful spells before Blain (the game's healer) can heal them.
* 2nd: The unicorn of all bosses is this. It has attack down and speed down, both spells that serously disadvantage the heroes at this point, it heals itself, and the heroes don't have powerful magic yet. Worst of all, the healing statue works only once. You seriously have to make the dungeon more than once. There is also the fact the dungeons are harder to beat at hard mode.
Dragondawn:
* The Guardian, who lampshades it even. He and his two helpers heal each other constantly and at least half of the heroes' attack do little damage.
* The last Dragon will trash your group unless you have the Excalibur-sword.
The Legend of Devils / Heroes:
* The two birds are no big deal in the normal game, the "Hard" mode included. The Advance Version (starting with the second) has however both birds calling another one. Worst of all, you won't be healed in-between and cannot teleport your party away yet. Mitigated in later versions where the surviving bird flees once the other one is defeated.
* Don Miguel and Esrah are the first bosses with insane powerful skills.
* While the Undead Knight is weak, he has three Mana-Mummies, who could rip the party to pieces. The Advance version even gives HIM powerful skills.
* Harry, who combines the two above with his high healing skill.
The Time of Bloody Sake:
* Azura. She has an all-draining spell and the second battle has her possibly doubling the characters. Somewhat mitigated in version 1.1 where she got replaced by another boss in the second encounter.