This fanfic got boring, now you're making Cedric trust Bron, you can see that he's getting attached to Bron even though he doesn't teach him anything and distrusts him, goodbye!
Brom was shown to be pretty useless in the book. He even admitted to it, though the reason for his weakness wasn't stated explicitly AFAIK. His exact words were 'every time I reach for magic, it gets a little harder' or something like that.