It's not even an issue of the final boss being too hard, but everything else in the game being way too easy. Every mechanic required to beat him is something you're rewarded for using in the rest of the game, but since you're never adequately challenged on the default difficulties it feels like you're just being punished on the final level. The only other "test" the game gives the player is the bladewolf fight, and enough people complained about the "difficulty spike" there as well.
I disagree that it's bad game design, though, because the final boss does require you to use all the systems in one way or another. Just think of him as the graduation test for the default difficulties, because everything you do in that fight is vital on Very Hard and Revengeance.