One way Tobirama can beat him is to use his Stamina to his advantage and try to outlast him. Mutually Exploding tags could potentially rip away layers of Itachis susanoo. And let's be real, no Kage level opponent who has experience fighting uchiha and has plenty intel on them is going to fall prey to genjutsu. He's not going to look directly into Itachis eyes, neither is Amaterasu going to work because of Tobiramas great sensing ability and body flicker or FTG which allows him to easily evade it. I doubt Susanoo will ever touch him, it's not fast enough to hit such an opponent. Water style > Fire style and Itachis water style isn't on Tobirama's level.
Itachi will lose in CQC due to FTG and speed advantage in tobiramas favour and the risk of getting marked. Honestly thinking about it, Tobirama may well be stronger than Itachi.