The Plasma Juggernaut is a monster created by the Mad Alchemist Edmund Gareth. It is called such because it's a combination of plasma energy and and golden material (alchemists are all about gold anyways). Oh yeah, and because this is an alchemical creation of material, it can qualify as a golem of sorts. It is non-playable boss of Brutal Fists.
Plasma Juggernaut is fought in the finals and just right before you face Edmund Gareth, the winner of the Brutal Fists tournament will have to face this thing first. It's a hulking abomination that will break you worse than Bane.