Well, we don't know too much about Muu other than his incredible sensory abilities and his amazing invisibility jutsu, making him and his chakra practically impossible to detect, and his dust release. I think that A could win, but only if he is able to fight Muu head on, and because of Muu's invisibility jutsu, I think this wouldn't happen, as A isn't considered a good sensor, so my bet goes to Muu dust releasing A for the win.