Walkthrough
At the end of Route 9, you will notice that the gate to Spikemuth (where the next Pokemon Gym is located) is closed and is causing many Gym Challengers to get angry. Marnie will quickly appear and will tell your trainer that she knows of a shortcut to get into the city, since she, in fact, was born there. However, before allowing you into the city, she will want to challeng you to another battle. After defeating her, she will give you Piers' League Card (the Spikemuth Gym Leader) and then she'll go investigate why the gate is currently closed.
Pokemon Trainer Marnie: Level 42 Liepard, Level 43 Toxicroak, Level 43 Scrafty, Level 44 Morpeko