I don't know if this has already been mentioned, but I'm pretty sure that Dullahan from Golden Sun the Lost Age was impossible to beat. The sucker could get 200 hp back every turn (So you couldn't get into a battle of attrition), could randomly summon Charon to do massive damage to your entire party, and had, among various nasty psynergiey attacks, something called "Formosa Sage", which did a very large and nasty sum of damage to a single character, usually enough to kill them if they were trying to summon anything.
