Deus Ex: Human Revolution. I got to this part in the game where I absolutely could not beat a boss. Consulted IGN and learned that the only way to defeat her was to have some ability that was only gained at some prior point. Unfortunately I didn’t have any save points prior to that, so the only way I would be able to defeat that boss would be to completely restart. I just kind of noped the game after that.
It was not an optional boss, sadly.