Why can't I use Uber Pokemon in Random Matchups if others can?

Question

I walked up to the lady to the right in the PokeCenter on the upper floor in B2, and I selected Random Matchups. In my party I had Mewtwo, Victini, Infernape, Darkrai, Haxorus and Rayquaza. It wouldn't allow me to enter because of the four legendaries. Then, out of curiosity, I watched some Battle Videos when talking to the same lady, and I saw some people using Genesect, Kyurem and many more legendaries. My question is: HOW DOES THIS WORK AND HOW WILL I BE ALBE TO USE LEGENDARIES MYSELF?

Answer 1

They were not using Random Matchup. The facility was Colosseum, where you can use any Pokemon, no matter if it's overpowered like Arceus.

Comments

oh. how do i use colosseum?