The Metal Frontier, last level of Glitchy's Adenture, is a dimention created by Lord Fear to help SMG3 with his plans. In there, SMG4, along with Ganoll, Clo', Dowser, Peach, and others, must fight with the 2 bosses in the area, SMG3 and Lord Fear. SMG3 tells SMG4 that it's too late, and soon the Earth will become metal. (literally, everything and everyone will become metal.) SMG3 then uses a power to clone himself, to help in the fight, But accidentally, instead of helping him, the clone helps SMG3. The battle, only SMG3's clone will fight with SMG3.
making it harder. When the batle is over, SMG3 says that he shouldn't have done that, because now all the dimentions will become metal. Then, SMG3 suddenly becomes metal, and the ground slowy become metal. If the player get in contact with it, he will lose, and will have to fight SMG3 again.
SMG4 rush to the factory, and there, he will fight some 900+ hp enemies, then he will fight the Final Boss.