My lil brother just took a swing at Dandarian in Final Fantasy 9, right after when his party has returned from Cleyra. It's a hard boss, considering that you're on a relatively low level with a low variety of attacks, plus that the clock is ticking. It can cast Doom, hit you with the book case, but worst of them all, unleash a paper storm at you that decimates a little more than a third of your HP. Alone, it's not that big of a problem, but since he likes to make that move several times in a row when the battle is closing to an end, then the chance of Game Over will suddenly become high.
We took it on the 5th try. Probably not the hardest boss of all time, but over the years it has earned my respect.