What you need is a Good Physical Attacker, and PRESICION. You need to move the pokemon around ALOT. I take advantage of the Pause that happens when you move your pokemon.

i used a Dark Raticate, and if you use Raticate, consider using Hyper Fang, Swords Dance and Focus Energy.

Next up i use Pichu to lower the defenses of each pokemon for Raticate to do more damage and use Sweet Kiss for some confusion damage.

Lastly, i had a bait-ish Pokemon Sylveon which whatever Resistance Potion it had, it helped me decide which pokemon to kill first.