Randomus, I think he is ok? Unless you have a very good team, he will catch you offguard. I did manage to beat him catching a Bouffalant, he did a lot of damage with Mega Horn (OHKO Malamar) But Luna, in the other hand, she is quite too easy. My team wasn't the best, the only thing I knew about her was that she uses Dark type pokemon: defeated her at first try (I guess this is cause the type itself, nothing more) But Charlotte, yeah... she was a pain, being not able to return to Reborn City was like death to me. And I manage to beat Terra at the first try
Dont read if you haven't beat the game yet