There are three ways I can think of at this point in time:
1. Mount Lanakila
You can get to Mount Lanakila by flying or through Tapu Village, on Ula'ula Island. The Pokemon that spawn on the mountainside or mid-point are the highest leveled wild Pokemon out of everywhere else, at levels 42-45. It could be useful for training lower leveled party members, but not so useful once you reach the 70s.
2. Hau'oli City Battle Buffet
The Battle Buffet is situated in Hau'oli Mall, just right of the Pokemon Center in Hau'oli City. It's straight ahead of the entrance. It costs 1,200 P to enter but once you do, you have 10 turns of battling to defeat Pokemon and gain EXP. Unfortunately, it can only be done once a day. However, the Pokemon are a little bit higher leveled, at around 50-55.
(Also, spoiler alert if you haven't finished the game yet!)
At a point in the post-game, you'll eventually be able to catch the Ultra Beasts. When you're able to catch Guzzlord [location will be specified in-game], bring the Pokemon you want to train with you (it should be around level 60+) and teach it a move effective against Guzzlord, preferably a Fairy-Type one as it's x4 effective, and continue defeating Guzzlord, over and over. It infinitely respawns, so continue to grind it until the desired level. It's to my belief that Guzzlord gives the most EXP for a wild Pokemon, and at a respectable level too.
Other Tips:
Make sure to put the Lucky Egg, obtainable by speaking to Professor Kukui in his "lab" on Hau'oli Outskirts after beating the game, on the Pokemon you want to train. Other than that, happy grinding!
