For the final battle I believe it is required that you attack her with the Balmung before she starts turning colours and you are able to attack. (which triggers the "where did you get that" line she says.)

 

Also the first time I tried to fight her I forgot earth runes and dident have earth spells but she did switch colours even though I never attacked with an earth spell. (though it did take a while) maybe you could have some one double check it. Maybe it only matters when it switches styles, regardless of type.

I concur with Maxmax - the mother DOES change attack style *eventually*.

Thanks so much, we've edited the guide with this information!

Feel free to PM me with any questions or concerns.