I'm quite sure there is a rule that keeps Superman from joining Batman into fighting anyone when there is prep since it's against the principle of having Batman being the main person battling.
Well, theoretically he can amass tech, built a robot army or make temporal dupes of himself, but I'm not quite sure about the last one since it may count as an extra character. The forum rules are quite pitted against someone who uses armies to fight than someone with more battle-oriented ability -it's a vs forum afterall-.
One way or another, I think Kang clearly has the upper hand if he hand picks a huge team of Anacronauts and sends it against the Corps.