I don't know if I'm just underestimating Mei, but I think she solos mid diff. Not low.
As for Kisame, it's actually a stomp-low diff.
Together, it's a complete and utter stomp.
It's a low-mid, I think it's low because:
the location gives her a huge advantage if it's just Mei. She can put up her mist and if they try to attack her close range which is what they need they will either be melted or die because of acid.
Now, Since Mei can adjust the temperature without melting/hurting herself. One could say that Suigetsu could just liquefy himself but that would only be down fall for him, Because if he turns into water Mei can use her Lava style which will cause the water to solidify so he can no longer move. Every suiton he tries to use on her will be countered by her own suiton or lava style.
It's even easier for her to take Mangetsu down, Once he puts up his mist for silent killing she can just use boil release and he will melt.
Water gun technique won't be of much help for the both of them because once her mist is up they can't see or sense her because neither Suigetsu or Mangetsu has shown a good enough feat to be able to know where Mei would be if she uses the Hiding in Mist technique.