Intel, location and starting distance?
With this characters it could completely chance the outcome...
Deidara wins with medium to high difficulty. The biggest challenge, I'd believe, would just be locating Mei. That coupled with strong Suiton on Mei's part, could waste a lot of clay up. Though I don't see C4 being countered.
C1 ain't doing shit to a water dragon. It'd take C2 or higher to cancel that out.
The samurai bridge, they can both use all of there ability's except for deidara's Nano bombs. (Sorry I'm late)
Mei can use everything she knows.
75m