Got my way up the ridiculously tall tower after getting a nasty surprise on floor one that I could use only magic. Carefully managed my skills and learned new ones to get there, collecting the rare items on the way and not gaining any experience except towards learning new spells. Finally made it to the top floor and got the super-item. Then left the top floor and encountered the boss. Started nailing me with level three spells. I did okay against them with all the anti-magic stuff I was equipping, but still hard hits against me. Set up my defensive spells and the like. Then found out that the SOB was only susceptible to one element at a time. This element changed whenever he was hit by magic, including the bloody "see what his weakness is" spell. I tried guessing for a while, just using whatever the characters had the best spell for, but with eight elements to choose from and 50000 Health points, I was getting no where fast, especially because I found out that a few elements would heal the bastard at any one point in time.
I had no other options, but after a bunch of experimentation while struggling to survive, I found out that two of my characters had a single non-elemental spell each. Drain. However, said drain spell couldn't ever do more damage than the character had to heal, so I ended up having half of my part beat on them while they damaged the boss (to the tune of 800 points of damage combined per turn).
After a nice long grind down of his health, I killed him. Then he used Ultima as a dying attack, doing double my toughest character's max health to the party. Game over.
Tried again, found out that I could use berserk on him, so he wouldn't use any magic. Whittled him down again. Still cast ultima.
Final Fantasy III on a Supernes emulator (which is actually FF6. I later beat him by draining every last one of his magic points, which he had more of than health).