Numerous possibilities exist. Though naturally as it's an opponent he has in idea about the most rational thing to do is get a grasp at his opponents weaknesses via the Kage Bushin. In terms of actually countering it; he can simply evade it (if you'd be kind enough to show off its scale? I don't know how impressive it is.) which won't be a challenge for a man with speed of his calibre, or Hirashin. Or he can simply disperse it with a Suiton Jutsu, or a Paper bomb (a feat even Sakura was able to complete.)
I'd like to ask you; what're both him and his summon's counters to Suidanha?
And off bat the clones all go poof with this:
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Huge poison cloud that almost instantly fills the area. And the difference between his poison and every other poison user is... lol... You do not have to breathe it or be cut to be affected! Fun fact; if a poison is strong enough mere contact to your skin is enough to get you infected.
Unlike Minato who has his FTG prepared and ready to use off bat, Tobirama does not have that luxury. He first have to mark something to use FTG basically steps has to be taken. Fact! Then on top of making clones/marking they then have to evade. None of that is fast enough to get out of the range and not be affected.
Suidana...lol... Is easily evaded or dodeged no diff. Hanzou is also a speed demon ya know! Then we have the many use of Exploding Kunais that will be utilized and spammed. Hanzou also has the ability to use explosions when ever he chooses with ease.
Then we have the close combat which he gets raped in. lol I can go into further details.
Speed feats of said poison, and show me where it needs to make contact at. I believe it needs to enter an open wound or get in the opponents respiratory system.
No poison does not have to use only those methods to affect you. Where did you get that from? And look up if you are clueless to the AOE and speed to took to fill said area.