He would wear a one-piece swimsuit designed for fat women. (Maybe one of Barb's) But he wouldn't last long, being around so many bare chested men, soon the voice in his head that says "Christian is gay" would come back and he would have to escape.
What would be his theme?
Autism. Chris thinks his autism gives him special abilities.
How would he plan out his evil plans?
He would just go out and blatantly commit crimes, but if anyone tried to interfere, he would use his autism as an excuse to get away with it. But all his crimes would be petty crap, like vandalism, loitering, trespassing, maybe robbing a video game store.
What would be his costume?
How would he dress his henchmen?
He probably wouldn't be able to get any, since they would need to be female, 18-current age, not black, b-cup minimum,-
How would we act around other villians like Joker, Two-face, and Poison Ivy?
All those male villains are JERKS and he wouldn't want to talk to him. The female villains would generally avoid him, but occasionally trick him into doing stuff to their advantage, like getting him to go rob the Game Stop to distract Batman while they are quietly breaking into a bank vault or kidnapping a scientist or something.
How long would it for Batman to beat the shit out of him?
Ah, well it's not that simple. First Chris would play the autism card. If that failed, he would shit himself so Batman couldn't touch him without risk of getting it on himself.