This is really cute! I'm sure things will get rolling once we're properly introduced to the rest of the main cast, but I kind of hope this continues on for a while longer as an absurd slice of life.
My guess is moreso they knew her weakness was fire (since that's what they traumatized/tortured her with) and just picked a decent, mid-ranked fire-based Hunter that could be easily persuaded - at least, easier to persuade/control than an A rank or better would be, I suppose