I like the way Capcom's Rival Schools games approach "unwinnable" fights, in that they're totally still winnable, but improbably difficult, especially for first-time players. In that, you're typically given a very limited amount of time with the opponent's AI jacked all the way up, and health/damage in its favor. You lose, the story progresses as usual. You win, you tend to get a little "how did you do that" story exchange and/or immediately warp to the final boss. Obviously, that would require some tweaking for a game where the main draw is re-telling an established story, but again, typically only players who have mastered the mechanics would see it.
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