When I say gets wrecked I could be exaggerating. It would do damage, but Manda very well may go underground while shedding his skin. Thankfully Tobirama is a sensor and would know where everything is and could take them out via Suidanha and if he had time, he could use Edo Tensei then Gojō Kibaku Fuda to destroy and obliterate Yamata no Orochi or anything bar Rashomon Gates. Tobirama and his clones can use Goshun Mawashi no Jutsu after marking Orochimaru with a Hiraishin mark for a plethora of different strategies. I don't see what Rashomon could do to help Orochimaru here, but elaborate if you wish.
OT: I wish more people liked Ebisu.
I'm not going to be dedicating a lot of my time to this, primarily for the reason I previously posted. But you overly rely on a strong, yes, but avoidable water technique that isn't an end all be all jutsu. It's not something like a Chidori spear, thus far shown, that can and will cut things such as Manda in half. It might leave some nasty damage but that's really about it.
You're suggesting giving Tobirama Edo Shinobi to take down Orochimaru? Give him an equal amount of Edo's. A Hiruzen Edo or watered down Hashirama. Orochimaru'll still win . And, if Orochimaru happens to be marked he can easily escape this scenario via oral rebirth. Or, hell. Taking off a piece of clothing that was marked. It's not like that's an impossible suggestion.
Rashomon basically works as a last minute defense against the suiton you rely so heavily on, while Orochimaru himself could evade it. It's simply something else he has in his arsenal to further counter the jutsu.
All in all, Tobirama has one real jutsu to do damage to Orochimaru and that would rely on Tobirama getting into a perfect spot to unleash it. Even then Orochimaru could [debatably] Counter it with his teleportation jutsu, hidden mole technique, and if Orochimaru gets HIS hands on Tobirama you realize he could ruin his day with the five pronged seal? Something that would obliterate his chakra control.
You're under estimating Orochimaru far too much. But, as I said at the start. This is all I'll give ye'. It's a tad incoherent but I think you'll get the general idea of how Orochimaru's abilities can be used to counter Tobirama's [ONE] lethal attack.
