The later of these are significantly harder than the main game, and some even pit you against multiple bosses at the same time. (hardcoregaming101.net)
My only major complaint is the final level which is too difficult mostly as a result of long, tedious fights against multiple bosses with excessively long health bars. (thevitalounge.net)
Throughout the zones you will face multiple bosses, some familiar and some new. (gameluster.com)