Clawmark Hill.
An area in the deep forest areas of Alola, Melemele Island.
The arena has no trees, and is an Arena made of rock formations. It's an area for training and getting stronger, the training and fighting only happen at night. The training is not fatal. 
 
This is only for Wild Pokémon, no humans or trainer owned Pokémon.
unless the human is wild, and live is if they were a wild Pokémon, then they're allowed to train here along with wild Pokémon, But that's extremely rare.