Dullahan, Golden Sun: The Lost Age.

Yeah, if you set up your team right and let half your characters die, you can summon rush him fairly easily, but otherwise this can be one of the hardest fights around.

And, while I'm on GS bosses, the Dragon of Gaia Rock, also from TLA. Normally, he's only moderately hard, but try taking him on without all the light reflected on to him. If you go straight to the boss, he'll fully heal each turn, forcing you to try a one turn kill on a high HP boss. If you reflect one beam, he won't fully heal, but he will heal enough to make the battle a contender for hardest of all.