Of the games I have played, its probably Killalon from Lost Odyssey: Seeker of the Deep.
To beat him:
(1) be level 99 and max out your characters through insane grinding.
(2) delve to the bottom of the deepest dungeon in the game, twenty-six floors of the toughest monsters in the game, no save points.
(3) walk into his chamber and discover a giant fuckoff robot in a fantasy game piloted by the LO equivalent of a Goomba.
(4) begin the fight. Every round, he auto-heals more damage than your team can actually dish out with the most powerful weapons in the game. By the second round he casts immunity to magic, and in the third round he summons an asteroid that kills everyone.
(5) sigh.
How to win:
The only remotely feasible way of defeating him is by going in with 1HP, and using a bizarre reverse spell that dishes out a HP damage value that is (YOUR MAXIMUM POSSIBLE HP) minus (YOUR CURRENT LEVEL OF HP).
If you can do this for the entire first round, and through pure luck get a MISS from his attacks with at least three party members, then he will summon the superasteroid that causes instant death. If you are VERY lucky, this may MISS one of your team who can then resurrect everyone with low health and if you can hold out another round and his magical immunity wears off, you can use the HP damage spell again to defeat him.
Even maxxed out, at a point where you can kill the real final boss in a single round, this strategy only has something like a one-in-sixty-four chance of ever succeeding as it depends on the boss's manouvers missing you successively. The strategy was only discovered months after the game came out, I believe.
I've maxxed Lost Odyssey and all its DLC, but for the life of me I couldn't defeat this monstrous boss.
