As for the fight.. Could go either way at a high difficulty. I don't see Tobirama not making shadow clones when the mist is up to try and out flank Mei in it and discovering the properties the acidic mist holds that way.
His new suiton could also cut her in two. I'll favor Tobirama more often then not
I'm going with Mei he scale of her jutsus would ring out/ kill Tobirama. His form of ftg is inferior to Minato's and his fighting style doesn't necessitate he tag things other than people.
Water Encampment Pillar would negate the harm from Tobirama's suiton.
The problem at the moment is that Tobirama's arsenal isn't large enough yet.