10. Mael Radec - Killzone 2
The final boss in Killzone 2 is an infuriating slog for several reasons.
First and foremost, even getting to Radec requires you to spend ages clearing out the endless fleet of Helghast in Visari Palace, who eventually bring high-powered explosive weaponry to the party.
It represents a major difficulty spike compared to the rest of the game even on Normal difficulty, and if you've got the stones to try it on Hard, you can spend literally hours trying to make it to Radec.
As for Radec himself, he's not only bestowed with heaps of health but can teleport around and deploy cloaking, while attacking from afar with a machine gun and then up-close with a knife.
It's a boss fight so exhausting and morale-draining that many just called it a day and watched the ending on YouTube.
It's not fun, it feels cheap and it's agonisingly drawn-out for bad measure. That feeling of finally topping Radec sure was something, though.