Laying aside optional bosses, the Elder Wyrm from Final Fantasy XII is a huge shock to the system in an otherwise leisurely game, and a lot of players really struggle to get past it.
Though, technically, you can get past it, although doing so is even harder unless you're aware of a certain engine exploitation trick.
I don't know that I would call Lost Odyssey easy, but the two hardest bosses in the game are both on the first disk: The Grilgan, a huge griffin-like creature, and the Bogimoray, a huge electric worm.
The Grilgan is hugely overpowered, but very straightforward: you attack as hard and as often as you can, and if it doesn't choose to do its superattack more than once, you may survive.
The Bogimoray, however, is just nightmarish. It has an appalling amount of health relative to your damage capabilities, and can charge a superattack that deals ridiculous damage and paralyzes your whole party. And when you finally kill the thing, which so far has made for one of the hardest battles in the game, another one immediately spawns, at full health, with its superattack fully charged. It's so ridiculously unfair. No other boss in the game is as frustrating.
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