Actually for Radomus the easiest way to defeat him is to have your Gardevoir use imprison on the first turn. Then proceed to use calm mind as much as you want and then sweep.
If your Gardevoir has the moves
Shadow Ball
Psychic
Imprison
Calm Mind
Radomus's pokemon won't be able to hit you with shadow ball, wont be able to set up with calm mind and they wont be able to use psychic