Oh, a more recent one... the final boss of Radiant Historia. All the bosses in that game have... issues (generally with being hard-hitting damage-sponges that ignore all the tactics you've been taught to use throughout the game), but the final boss is... brutal. To me at least - I might just be severely underlevelled, because the feedback in that game is harsh (some enemies go down in one hit, and others in the same area basically one-to-two shot you).

You start out with a battle against a different foe, who requires a completely different set of tactics. And then can't swap out your party. First phase isn't bad. Just tedious. Second phase... it has an attack that puts your full party to sleep. And then an attack that, unlike every other attack in the game, doesn't wake sleeping targets. And you only wake up if damaged or have a status cure spell/item used on you. So, if you don't have an item to resist it, you die.
And then the third phase has a full-party attack that dealt more damage to each of my party members than they had max HP.

But, the worst part is... if you lose... you have to do the whole thing again. Including the pre-final boss. Which takes a bout a half hour to get through to get to the third phase, because everything is so hp-sponge, and so damage that you have to spend half the time healing, and making sure to MP regen. And if you use the "retry a battle" mechanic resource that they've given you... psyche, it sends you back to the save point before the penultimate boss, only you're able to skip the cutscene when you retry because you've already "seen it". Which no other battle does. It's frustrating, and, to this day, I have never beat it because it's just so annoying to try and figure out what I've been doing wrong.