Luca Blight from Suikoden 2. SPOILERS AHEAD...
Most boss battles in RPGs take just the one go. One team, one fight, done.
Luca Blight gets ambushed by an army, shot with arrows and then takes on 3 teams of your best warriors. All 3 teams can have up to 6 characters in them. None of these teams kills him. He wanders off into the forest, laughing... There's a trap - He gets shot with a bunch more arrows from your army. Your main character takes him on in 1 on 1 combat. You win, and he dies laughing.
His final words - "It took hundreds to kill me but I killed humans by the thousands. I am sublime!!! I am the true face of evil!!!!"
The battle theme is quite awesome, too - http://www.youtube.com/watch?v=MX8fKtjpr_E
