Sephiroth Battle (Kingdom Hearts) was the hardest boss i fought.
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From Dracoslythe
It took me a few trys to learn his moves and defeat him, and i was at lvl 76. Seph's sword slashes take away most of my HP when i was lvl 50-60 and with crappy accessories equiped. His Sin harvest move is really powerful since sin harvest makes my Hp 1 and MP 0, so i have to heal fast, or try to intercept his sin harvest. I also use aero shield magic and cure or health potion a lot during the battle. Seph also teleports a lot, so he's hard to hit. Plus, His sword is so long, and he swings it really fast. Seph also runs really fast, too.
Note: The voice of Sephiroth in kingdom hearts is Lance Bass from N'sync the boyband.