Actually, I can think of something worse. Since he's a machine life-form with an extreme bent of megalomania and power complex, I can imagine a worse punishment then being a grunt would be a lawn statue; a futuristic "Thinker" if you will. I can imagine Largo's hell to be sitting inert and unable to speak, but still able to think. He could fashion and formulate all the evil schemes and think dark and angry thoughts for all eternity, but couldn't move or talk or do anything about them. And his eyes would be open, just to make his life more boring. As a grunt, Surtur might be able to elevate him after all.