Boss fights are not easy, they are never supposed to be easy. They are meant to be challenging and fun, but some on the other hand are very difficult and fustrating.
For example, in the 2014 THIEF reboot there was the Thief Taker General, If you are trying to do a No Kill/KO run for the Moral Victory achievement, it is almost impossible. You know why? Because he fires explosive arrows at you which almost certainly kill you in ONE HIT. You try and crank the wheels to open the gate, BOOM! YOU'RE DEAD!!! If you somehow manage crank the wheels without getting blown up, there is a 5 tier lockpick. Just as you are about to unlock the door, BOOM! YOU'RE DEAD!!! It is so difficult that I ultimately ended up killing him using explosive arrows.
What's the hardest boss fight you've ever came across?