Guitar Hero 3: Legends of Rock.
The guitar battle versus Lou.
It was all about luck. Stupid game design luck. The thing with Guitar Hero is that it was perfectly doable throughout, having only one sudden but eventually manageable jump in difficulty curve. But then when it got to the end, the game didn't care about skill, ability, or whatever, it was all dependent on getting the right handful of attacks while hoping your opponent grabs the wrong handful of attacks. Every run was like a sadistic game of rock paper scissors with the deck stacked heavily against you.
You're playing along, and you will pick up powerups that will make the opponent's highway flicker, or get obstructed, or break a string. But it's an AI, they don't need to actually see the game, their coding only mandates that, if flickering, they have to miss a certain percentage of notes that still won't kill them, so it's completely ineffective no matter where you trigger it! You had to get the string break, and you had to pop it at a strategic spot where notes of that particular string were bountiful to drag him down, but it was a dice roll which string broke. So now you're not only hoping for the string break attack, you're hoping that it breaks the best of five possible strings. Hits the red string? You're good. Hit the orange? What a damn waste.
Meanwhile, the AI is programmed to get every possible sequence required to earn an attack, and they will throw it at you when you get your opportunity to get your attack, breaking your combo and losing your ability to earn that attack.
It wasn't even challenging. People say "oh you're just not good enough, git good, suck less newbie," but hell no, a 14-year old reincarnated Jimmy Hendricks with spastic tarantulas for fingers would get instakilled not matter how much coke he was hopped up on. The challenge wasn't the song, the venue, or even the sequence of notes. The challenge was all in that stupid game design.
If took me an entire afternoon just to get past that last damn badly designed boss battle. Guitar Hero, in my opinion, got a lot of things right. But that Lou Guitar Battle at the very end of Guitar Hero 3 was just absurd, frustrating, and cheats like Candy Crush.
And the worst of it all. You had to beat this level to earn Through the Fire and Flames by Dragonforce. And every one of my friends and coworkers wanted that song, but didn't want to go through the aggravating hell I endured to earn it. So I couldn't just leave the Lou Guitar Battle behind me. I ended up beating it repeatedly for the rest of them, too. And no, it didn't get easier. It was still a terrible trainwreck of cheating and explosive vulgarity erupting forth from my larynx.