How do you get azelf after beat the eliet 4?

After you have defeated the Elite Four and become the Champion, you will gain the ability to use Fly to any Pokécenter. Use the Fly ability on your map and fly the town called Hearthome. Next head all the way to the North of Hearthome city. You will see Route 210. Enter Route 210 and use the bicycle to go around the small lake with water, then head south. Keep heading southeast and you will find a lake with a little island in the middle. This island is called Lake Valor and you will need the Pokétch app called the DexNav. Then open the Dex Nav and click on the Pokémon you wish to encounter, in this case, Azelf. If you already have the Pokémon registered in the Pokédex, you can tap on its icon and select "Search." If you haven't registered the Pokémon yet, you can look it up in the Pokédex and then select "Search." Use the Dex Nav and a new encounter screen will appear. Once Azelf appears you have to find it using the soundwaves and shadows the Dex Nav App shows. Once you find Azelf, a battle with Azelf will start and then you can catch it with your Pokéballs.