Devil Survivor 2.
DS1's final bonus boss was a legit challenge
DS2's? No way. I did about as much preparation (But a lot more leveling, even if it didn't take nearly as long), but it was still an absolute joke of a fight
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http://img.imgcake.com/RegaroUlquiorrapngse.png
http://img.imgcake.com/drakeryn/regarojpgpe.jpg