Mismagius, I'm sorry to say, is a wannabe Gengar. There's nothing it can do that Gengar can't do better. Gengar's got higher Special Defense, Special Attack, and Speed, and literally the exact same movepool. So I don't think there's any reason to use it over Gengar.
EDIT: The only, only thing that Mismagius has over Gengar is Nasty Plot. (Creds to Your Worst Nightmare.) Here's a set to capitalize on that:
Mismagius @ Salac Berry
EVs: 12 HP / 244 SpA / 252 Spe
Simple, short, and sweet. The given EVs make the Salac Berry activate after 3 Substitutes. Shadow Ball and Dazzling Gleam are unresisted bar the Litleo line. Sub when you get the chance, Nasty Plot up, and destroy.