Well, the Marauders are supposed to be like fighting yourself with the only Reason you can kill them being because you aren't running on AI and have a larger Arsenal but from what I have seen of the Final Boss it seems like the guy is supposed to be like you are an Imp trying to fight against the Doomslayer. Which seems to be a rare instance of Gameplay and Story
Integration rather then Segregation. Alternatively the Dark Lord has been training on Chaos Space Marines (he certainly has the armour of one).
Punch-your-face-in levels of
Nintendo Hard, I officially crissen this Boss into the Trope "
Super Nintendo Hard".