Pokemon X and Y Walkthrough: Champion Diantha
Champion Diantha poses the final challenge. Unlike the Elite Four, she doesn't specialize in a single Pokemon type. Instead, she uses many different types of Pokemon with multiple move types. Her Tyrantrum and Aurorus are both weak to Fighting, Ground, and Steel-type moves. Hawlucha and Gourgeist share Flying and Ice-type weaknesses. Goodra also has an Ice-type weakness. Her Gardevoir can Mega Evolve, which raises its Special Attack quite a bit. It would be best to knock it out using Physical Attacks before it has a chance to do damage. In order to maximize weaknesses and minimize resistances from your moves, make sure to use Ice and Ghost-type moves. You should've already received TM 13 Ice Beam from Wulfric. For Ghost-type moves, you can find TM 30 Shadow Ball in Terminus Cave and TM 65 Shadow Claw in Glittering Cave.