Hey Toto, I know you've sent him to another realm and all, but Cronos doesn't know that, and Nightmare doesn't know that. Even in another realm, Cronos is strong enough to break through, except I figure it would cause more pain to the recipitent, along with the barrier. Quick Claw does not have enough psychic ability, nor the proximity to be able to do anything of the sort. This block works on both Clans and Elites, and keeps them from both sending and recieving mental messages from each other.
That's all, really. Seriously, I'm not trying to flame you - I still feel guilty of the other times - but really, Mewtwo's a purely Psychic Legendary, and a strong one, too. A Fearow is... Pretty common and is a Normal/Flying type. You see where the difference in mental abilities lie?