If he's comically stupid enough to come out throwing punches or trying to brawl, he'll be gassed out by the fourth round and get put to sleep with a combination of brutal and perhaps disfiguring punches.
He needs to be realistic about his lack of intelligence and fitness. Minimize all of his movements, throw absolutely nothing for at least the first half of the fight, just clinch for dear life, prop his weight up onto whatever limb he can secure a hold of. Do that for long enough, whilst blocking out the growing anger of the fans calling for his immediate beheading, making sure himself and his corner are applying enough psychological pressure on the referee, vociferously berating him each time he breaks them up, eventually the ref will get the message, and then hopefully, after a few rounds of this Joshua might be lucky enough to force Usyk to gas out before he does, or at least make his arms tired and lose their snap, and then he can take his one in a million shot at trying to knock him out.