Now you may ask, why hasn't she go into that when she was battling Warrior? Well, her ghost form is Ghost/Fighting, she learns a completely different movepool, has a different ability know as Tough Claws, both in ghost and mega form. Even though she might, but if you find out, he's has a move called "Foresight," meaning once use, he will enable Normal and Fighting type move be effective on Ghost Types. So, pretty much Warrior will win regardless if Nieve goes in ghost form.