Hmm, A good question indeed. Who do I think could beat superman? Well, here we go!
1. Loki! @JimTurpen made a good point. Loki is a cunning little shit, and he has control over magic. Supes doesn't stand a chance.
2. Doctor Strange! Another magic user. Enough said.
3. My character. Well, she's actually friends with him, but yeah, if she was angry with him, she could knock him on his ass. 1: She's a magic user 2: 2,000 years of martial arts training. 3: Lots of other reasons I don't feel like listing.