Am I the only one who can't use them? I just battled someone with all Ubers which includes Arceus, Palkia, Rayquaza, Mewtwo, Girantina, and Shaymin. But when I use even a Rotom it told me to change my legendaries. And my Pokemon are legit too.
That happened to me too! I onced fixed it. Have you beat story mode? I think you have to beat certain colloseums certain amount of times to unlock the use of certain legends.