Dullahan in Golden Sun the Lost Age
Not only is he a secret boss you can only get if you found all the Dijinni in the first game and the second game, but he is also twice as powerful as the actual final boss of the game.
He also had a spell that sent all your Djinni into recovery mode. If he did it within the first few turns it was pretty much GG as he instantly killed your entire party. But all the work was worth it in the end, since he was guarding the best summon in the game. But at that point you probably only had the final boss to use it on.
I even remember his battle theme sounded like you were biting off way more then you could chew. I remember he instantly murdered my entire party in 1 turn and I went "Wait what?"
Jacco said:
The Archdemon in Dragon Age. For some fucking stupid reason I decided it would be a good idea take him on with Sten, Alistair, and Dog. I still haven't beaten it to this day.
Try fighting High Dragon first, you use the same strategy with the High Dragon as you do with the Archdemon. Or try killing Flemeth first.
The only real difference between the High Dragon and the Archdemon are that you can summon npcs and hero characters to fight the Archdemon with you. With the High Dragon it's just you and your 3 party members against a giant 10 story dragon.