The final boss of Arc the Lad 2 (I think it was called the Dark One or something like that, but I haven't played the game in more than 15 years so I don't really remember anymore) was pretty ridiculous.

My team was massively overpowered and over-leveled because I went and played through the Monster Arena disc before finishing Arc the Lad 2, and I discovered how to clone items including the most OP equipment and the items that would just level up your characters, using save data manipulation if I remember right. And even doing all that, it still took me two hours to beat the final boss.