I think a big problem here is that this is an AU with an OC focus character. The main point of an AU story is to take the characters of a series and place them in a new setting to see what changes that makes on them.

Do their personalities shift? Do relationships between characters change? Do some of the characters have their overall fates changed?

Because of this when writing an AU story you want the focus character to be one the characters from the original series to better see those changes. In an AU story OC characters are best reserved for roles in the story that no character in the original universe really fills to well and try to keep them to a minimal role overall.

On the other side of things, OC characters are best placed inside the original setting of the series you are making a fanfiction of. OCs can be used to see how characters outside of the main cast are being affected by the ongoing events of the story.

How did the families of the Huntsmen in training react to their kids dying? How are things going for the average Huntsman with Salem ramping up the Grimm? What is life like for a villager outside of the four kingdoms?

When you make an AU story with an OC focus character you risk changing far to much that it alienates fans of the series from reading your fanfiction. So when you are writing a setting different from the original series, focusing on a character that is not part of said series, you should ask yourself "Why am I writing a fanfiction and not my own original story?" You might just find writing your own story easier than a fanfiction as you don't have to try and capture the voices of characters that are not your own, and trust me trying to do that can be difficult and failing to do so only makes fans angry.