Okay, so it's certainly not the hardest hardest game in the world ever, but Z.O.E 2 was insanely hard at times for hilarious reasons.
One fun thing about the game is being able to play through with various unlocked mecha, but if you played with the strongest mecha, one boss battle became nigh-on impossible because you were too good.
At one point in the game a friend gets 'possessed' and attacks you, so you need to stun them, but you're so powerful that one slightly mistimed hit will probably kill her.
Reload...
Kill...
Reload...
Kill...
AAARRRGGHHHH!!!