I'll say Deidara cause Mei lacks the ability to keep Deidara from getting on the ground. It's easy for Deidara to get up in the air and fight to his advantage with just some C1 bombs. After he's in the air, Mei hardly stands a chance. Corrosive Mist might have an effect on Deidara's bombs but Deidara can send a barrage of bombs to Mei. Chances are she won't have enough mist or speed to stop Deidara's attacks.
Assuming his arms aren't melted by then :|You must be registered for see images
He can just fly above the mist... or use his bombs to dispel the mist
~ Deidara solos