I'd give this to Orochimaru high difficulty. Without edo tensei, Tobirama's offence is quite limited, none of which is enough to put down Orochimaru. Tobirama's best bet is outlasting Oro but Oro can fight for quite a while as he himself is quite the stamina beast. Oro's snake summons could give Oro openings and his sensory abilities as well as reaction time could allow him to use attack prevention whenever Tobirama strikes. Tobirama will eventually fight at close range, allowing Orochimaru to potentially poison him.