at (one's) own game

Using another person's strengths, techniques, or tactics against him or her. Typically used in the phrase "beat (one) at (one's) own game." This team relies on their defense, so let's beat them at their own game and deny them any opportunity to score. I did start that rumor about Tiffany because she's always gossiping about me—it was time to beat her at her own game.