The Path of Solitude is an optional set of challenge battles that you can undertake by speaking to Warden Ingo at the Jubilife Village Training Grounds.
As its name suggests, you will battle with just one Pokémon, of your choosing. To win, you (usually) need to defeat a single opponent. However, this Pokémon very often has a type advantage against yours, making it extremely difficult.
How to Unlock the Path of Solitude
The Path of Solitude isn’t unlocked straightaway. After downloading the Daybreak update, you must also be at the postgame (Mission 19 or above) and you must’ve completed Request 102 (the actual questline starts from Request 95).
Once both conditions are met, when you return to Jubilife Village, you’ll automatically be brought to Ingo and introduced to his new challenges, which includes the Path of Solitude.
Benefits of Completing the Path of Solitude
For completionists, perhaps the most important thing is that you’ll receive a special stamp on the Pokémon’s Pokédex page for clearing the Path of Solitude with them.
Besides that, when you clear a Path of Solitude battle, you will earn some token rewards: Exp. Candies for the first clear, then Grit Dust thereafter. After clearing a certain number of unique battles (10, 20, 30, etc.), you’ll also receive Exp. Candies.
There are also a bunch of requests, starting from Request 108 that award Seeds of Mastery and Exp. Candies for clearing the Path of Solitude with specific Pokémon.
Tips for the Path of Solitude
Due to how the opponents are chosen, there aren’t any easy ways to win Path of Solitude battles besides grinding for Levels and feeding your Pokémon Grit items to boost their Effort Levels.
If you don’t mind hurting yourself, you can also purposely damage yourself by dropping from the sky (but don’t fall too high or you’ll faint). While you’re damaged, Blissey will face you and dance. Wait for your health to naturally recover, at which point Blissey will begin running, then toss a mon their way.
Otherwise, you can buy Exp. Candies from Bonn near Galaxy Hall, but they are rather expensive.
For Grit items, the easiest way to get them is by defeating/catching alphas or mass-releasing unwanted Pokémon. What you could do is catch lots of them from Massive Mass Outbreaks, then release them.
Besides that, don’t forget to speak to Zisu (near Ingo) to learn new moves for your Pokémon and/or master their moves. Since most of the opponents have a type advantage, your Pokémon’s natural moves often won’t deal much damage. But if you’re lucky, Zisu’s moves may offer an opening.
Finally, if you’re aiming to complete as many Path of Solitude battles as possible, it’s a good idea to plan ahead. Namely, if your Pokémon is from an evolutionary family, it’s smarter to train an unevolved one, clear the Path of Solitude with them, then evolve them and work your way up. Eg. don’t go straight for the Gyarados battle, start with a Magikarp first.