Dullahan, a recurring secret boss from the Golden Sun series. I found him especially tough in Golden Sun: Dark Dawn. He can just completely obliterate any strategy you devise thanks to some of his moves and the way they interact with the game's mechanics. For example, he can effectively reset all of your characters to their starting classes for several turns (which includes stat growths and spell lists), disable all of your summons, inflict instant death on your entire team, and much, much more. Oh, and he also gets like 3-4 actions per turn. In addition to that, he has an assload of HP. Just soooooooo much HP. Fortunately, his actions always follow one of a few set patterns, selected randomly at the start of the fight. He can start at any point [i]in[/i] said patterns (and the patterns are fairly long), but once you figure out which pattern he's using, it's possible to create a plan to circumvent his most broken moves. The reward is more than worth it, though. You're given access to the single most powerful summon in the game, which deals massive damage to all enemies [i]and[/i] revives/fully heals all of your party members (and reserve team iirc). It trivializes [i]almost[/i] any boss rebattle in the game, but Dullahan is most likely the last challenge you'll tackle in the postgame. However, even having the ultimate summon doesn't make any rematches against Dullahan much easier. He can still completely negate your ability to summon (or even use it against you iirc). [spoiler]It's been several years (6 or 7) since I've played Dark Dawn, so I may have forgotten some details of the fight. Feel free to look it up.[/spoiler]