Upon reaching the Champion's temple, Alder thanks you for defeating N and quickly accepts your challenge. His Pokémon have a variety of types, though some of them share weaknesses. His Accelgor, Escavalier, and Vanilluxe are all vulnerable to Fire-type attacks. Accelgor and Volcarona are weak against Flying-type moves, while Vanilluxe and Bouffalant both take double-damage from Fighting-type moves. They all attack with powerful moves, so taking too much time is dangerous. Strike their weak points to defeat them quickly.
When you defeat Alder, the two of you take the warp behind him to reach the Hall of Fame. He activates the machine here, and you and your Pokémon are recorded for the ages. The credits roll a second time, and you wind up back home in Nuvema Town again. Fly to Opelucid City and go east to reach Route 11.