Literally, planchar el traje means "to press or iron the suit". It is an expression used to convey that something passed like a steam roller over somebody, leaving his or her suit completely wrinkle free. It's a metaphor for a clear defeat. It is commonly used in sports and professional life. A synonym would be "to walk over".
¡Chale! Volvió a plancharnos el traje nuestro acérrimo rival.
Damn! our arch rival walked over us one more time.
Art by Banksy.