With his trump card, Skull Knight would solo with ease, due to his Behelit Sword being implied to be dangerous to the God Hand. Without it though, he'd probably lose, since he didn't seem to be too far above Zodd during the Eclipse.
As for Berserk Guts, he's probably rivalling Grunbeld and Zodd's apostle forms, though the limited time in Berserk Mode would leave him unable to defeat either. He may be able to beat Locus based on current feats though, since beyond his lance feat's physical impossibilities, Locus doesn't have much going for him right now.