The thing about Pokemon is the basic JRPG mechanics of it are so straightforward that there are very few solutions that work as well as Grind More--which, again, is time-consuming, not difficult. Almost all of the game's problems can be solved by Grinding More and there's rarely an occasion where you have, say, the right held item (since almost all of those are doled out at endgame anymore) and shuffle it in and then get rewarded.
Ultra Sun/Moon is a great example because I think a lot of the battles have legitimately clever elements. Guzma starts with his Golisopod, which is super bulky but also has Emergency Exit, and if you don't kill it in one hit it Emergency Exits right out to Masquerain--a special attacker with Intimidate, so you may need to switch depending on your team. That was a legitimately interesting tactic, but I never found a solution to it better than Grind More with, for example, Vikavolt. Beyond that you get Totem battles that force you to take things on in interesting ways or try new tactics, like Kommo-o being set up basically to shit on Fairy types that could easily stop it, but the solution is still the same. Grind More.
Making the game harder without fundamentally changing what Pokemon is (i.e., not adding something like timed button presses or whatnot) would require looking at a few things like
* getting rid of Switch battle mode (which you can do anyway)
* limiting resources in some way since it's not really that hard to potion spam your way to victory most of the time
* improve enemy AI so they actually switch out of unfavorable matchups instead of only switching out when they can literally do nothing (something with only Normal or Fighting moves vs. a Ghost)
* and so on.
So basically it'd have to run more like stuff like Battle Maison and such.