Wouldnt demons be effective fighters against Viltrumites since most of their weapons and powers would be magic/hell based and thus would easily by pass Viltrumite invulnerability? Not even mentioning most Viltrumites are no doubt dripping in sin so things like Hellfire should def. absolutely melt them like butter.
People keep acting like Superman cant be stopped without Kryptonite when magic and psychic powers have demonstrated to being able to affect him just like anyone else.