The final boss of Grim Dawn during late early access was the worst. The boss kept spawning overpowered minions and spamming powerful ranged attacks all the time. I think I died more than 30 times while killing him and I couldn't have done it if his health regenerated when I died. Luckily in games like Grim Dawn, enemy health regeneration upon death isn't common. And as luck would be, the boss got nerfed the next day or the day after that. So yeah, good times (not).
Also, not exactly a boss, but sort of. In Driver 2, in the third last mission, you have to chase a guy and destroy his car. The mission happens mostly on a mountain road where it's ridiculously easy to fall off if you try too hard - which you do a lot, since the target drives pretty fast. And like in every mission, it's also possible to wreck your own car, which also happens quite often. It's a pretty simple mission but it took me literally years to beat it, cheats or not. I think I finally did it without, but the mission was ridiculously difficult.







