What does SubCM Cress run as its mono-type attack? If it runs Ice beam isn't going to do much without a ridiculous amount of boosts. If it's Psychic then you can simply send out a dark type and wall Cress. If it runs both then it doesn't have moonlight, meaning you can be worn doing throughout the match.
It's not going to be worn down if you can barely damage it, and it's always going to have the Lefties recovery anyway.