Fanfiction writers here...?

You need a character that seems believable. As long as you parade your OC as someone that is, essentially, untouchable... she will be. The problem is that she will also be untouchable to the audience. They won't be able to relate to her on a personal level. Give her some depth to compensate for all of that ability. What motivates her? What is her major weakness? What topics make her feel insecure? You should also be sure that she's in an environment where that kind of power is balanced. Are your "bad guys" equally capable? If not, she would gravitate toward the overkill side of things. If no one lasts more than a minute fighting her, then you're going to be hard pressed to get a conflict going. Of course, the conflict doesn't haven't to be physical. I just umed you would go for that angle, given your OC's affinity for fighting. Speaking of which, have you considered concentrating her abilities in one specific area of fighting? Maybe she's good at martial arts and related weapons, but is an absolute klutz when it comes to one of the other fighting specializations. Anyway, that's the tip of the iceberg. I hope it helps. Good luck with your writing! :)