Gaberiel from Star Ocean 3. Lemme tell you, do not go in that fight unprepared cause he knows how to spam paralisis and an ultimate move called dragons. Dragons freaken hurt if you can't run and hide in a corner.
Megaman DS, Megaman Battle Network 4, Red Sun. Fighting him a Super Hard Mode after you've most likly used every chip ever is a pain. He learns from watching you fight and he can spam Giga Chips.
Same boss from above but in Part 5. He's not really a boss, but when he pops outta ChoasNapalm Dbl Soul. It's really unexpected and he'll spam MegaBuster and he's invincible.