Going from the basic, his Suiton not only has elemental advantage against his Katon, he's renowned as the most powerful suiton user in history (also one of the fastest in his generation, not taking into account FTG). Now, getting a bit fancy, FTG is more to what Itachi can hope to deal with, also taking into account that Tobirama won't have much issues marking him having faster shunshin. Also, on OP, knowing about FTG or even his Speed doesn't really make it a defense whatsoever, as Lee once explained to Sasuke, it doesn't matter if his sharingan can foresee his movements if his body cannot catch up to the speed.
And finally, getting a bit too literal, Edo's aren't brought back with full power, so feats against Sasuke might not apply here.
Again, all of this is moot if Tobirama doesn't have some sort of sealing jutsu, but seeing that this is the guy that invented Edo Tensei, I wouldn't be surprised if he has.