First step: train all your Pokémon to level 100. Mt. Battle's Areas 9 and 10 are perfect for this, also you can find rematchable trainers with level 50 teams at the Outskirt Stand, Phenac City, Pyrite Town, Cipher Lab, and Agate Village.
Next you should make sure you have at least one Safeguard user and two Protect/Detect users. Safeguard will make a joke out of the first round, and Protect/Detect are almost required if you want to stand any chance of winning the later rounds.