After you become Champion, you can talk to Guzma and battle him one more time.

Challenge Guzma

Go to the house to the right of the motel in Route 2 of Melemele Island. Talk to Guzma and he tells you to go Hau'oli City Beachfront.

Before you go, go into the bedroom in the back and you get TM87 Swagger.

Then you can find Guzma on the far end of the beach in Hau'oli City Beachfront and battle him. Pokemon Trainer guzma has a level 63 Golisopod, a level 63 Masquerain, a level 63 Pinsir, a level 63 Ariados, and a level 63 Scizor. You get 7560 Pokédollars for winning.

Afterward, Guzma gives you a Dawn Stone.