How to avoid writing a Mary Sue?

Ok I'm writing a Narnia FanFiction and I have a fear that my character will turn into a mary sue. ( a character whom everyone loves and has a dark past. Mary Sue's often save the main characters or are damsels in destress.) My character doesnt know how to fight (as most mary sue's do) and she is quite clever. she loves to pull pranks and isnt very tough. please tell me how to avoid writing a mary sue!