It's one of two things: Cerebrus is an OG 19 weapon or he can dual wield
Shanks' sword is prob an OG 19 weapon as well. Maybe it enhances or channels the person's haki much better
Base Shamrock is likely on Shanks' lvl or slightly weaker. But Omen buff + special weapon evens out the playing field if not tip it in his favor. Oda wrote Shamrock to provide a foil to Shanks, who's been his golden boy the entire time. Crazy times