i don't know if this has been mentioned before, but Lilith Aileron in the playstation version of Tales of Phantasia was really hard. Basically you could never get an attack in because she always countered everything with giant multi hit martial art attacks (with a ladle).
apparently, some magazine recommended fighting her at lvl 99, as well as fighting the last boss at lvl 70. Well i managed to beat her at lvl 80, after beating the game at lvl 64, but i could have done it at a lower level.
not really the hardest boss in existence, but worth mentioning
