I'm so sorry you are having trouble. And I thought a fistula would have gotten our son approved right away!
First, make sure to appeal the decision through the proper channels. Your GI nurse should be able to do this, but follow up to make sure it is done correctly. At some point, part of the appeals process is a "peer to peer" phone call between your GI and Anthem's MD. Even if Anthem says this will not change their decision, push for it.
Finally, someone at Anthem told me that my husband's employer could over-ride Anthem and instruct them to cover the Remicade. But it would be setting a precedent that my the employer may not want to set. When it really looked like we were not going to ever be approved, my husband contacted the benefits analyst at the corporate HQ. She talked to Anthem, sent all the paperwork through again, and told us to send a letter telling our side of the story.
I wrote a letter to Anthem describing our son's situation, how Crohn's has affected him, and how prestigious Dr Denson and Children's is. I can send you a copy if you want to give me your email address. It was just after they got the letter that they finally approved the Remicade.
I hope this helps! Let me know if you want a copy of the letter.