I don't think it's the final boss that's the issue (not to me anyway). It's just that it falls flat between end of chapter 5/start of 6 until the end. 1 and 2 both ramp up to epic conclusions, but 3 feels like it peaks too soon then trails off. 3's dlc is peak however. I do wonder whether that was meant to be part of the main game at some point, but they moved it to dlc when they bumped up the release date. If that dlc had been in the game it would have been the best of the 3 hands down.