Drakengard 3.
For those of you who don't know, Drakengard is a Hack & Slash RPG that plays similarly to Devil May Cry or something of that ilk if it were very loosely crossed with Dynasty Warriors. And the game isn't that hard.
It isn't that hard, until the end. The final boss is like no other boss in the game in that, rather than being an average fight, the game switches to being a Rhythm Game. Now you may think "Well that doesn't seem that bad, you can just get into it as you go, since Rhythm Games are pretty accessible." That would be the logical thought, since that's how Rhythm games work. You don't need to be good at one Rhythm game to get better at another, it's generally a case by case basis, and the boss fight starts out easy so as to ease you into the fight and get a handle on the controls.
So why is this a hard battle, you may ask? Because the rhythm doesn't go with the song and rather than being based on what you can hear, it's more of a visual ride since it's not very cohesive. It becomes about timing rather than keeping up with the rhythm, and this is made worse by the fact that the game's cameraman is apparently drunk and goes absolutely crazy.
And then, to make matters worse, the boss fight tricks you into thinking its over when it isn't by playing dialog while the final beats are coming (and considering the song is pretty long, there are no checkpoints, and you can't mess up even once, it has lead many, MANY players to find workarounds or just watch the ending on Youtube).