I will say Zeus in God of War 3 in Chaos Mode difficulty, specifically the third part of the battle, Zeus will create duplicates of himself and attack you from all sides, even in Titan difficulty you will still get hit no matter what so you just have to keep punching the clones with the Lion fists (I'm to lazy to remember the names) and with the Spartan shields and arrows, as well as replenishing your health with Gaia's hearth. Its a pain in the ass but doable in Titan mode, but in Chaos mode its just crazy, there is no way to avoid most of the damage so the only way to beat Zeus is by constantly replenishing your health and do random attacks and hope for the best, it becomes an endurance fight rather than skill.
I was also going to mention the Last Valkyrie in God of War 4, she is far more difficult to beat but she is not unfair.