With 145 unique battles in total, it would take a really long time to give help for every single battle. Plus due to the nature of the fights, you’ll basically be doing the same kind of things for each fight. Instead, here’s some general advice that should help you overcome those persistent Master Trainers!
1. No Need to Play Fair
At first, it may seem exciting to test your well-trained Pokémon against the so-called “masters”, but in general, the odds are stacked against you. Most of the Master Trainers have Pokémon with multiple max IVs and lots of AVs in key stats. So they’re probably stronger than your Pokémon, even if your Pokémon are higher Level. You could brute-force the battles, but with so many Master Trainers, there’s an easier way than that.
Against most of the Master Trainers, you can rely on the cheap and cheerful Toxic and Protect/Substitute spam. First, use Toxic to inflict deadly poison on your foe, then stall out the fight with Protect (which shields from all damage, but may fail if used repeately) and Substitute (which sacrifices 25% HP to place a decoy). Simply survive for 6 turns and you’ll win! This may take slightly longer if your foe can restore HP. All three moves can be taught via TM; please refer to our TM List for their locations.
2. It Could Make Alola Difference
The rules for a Master Trainer bout specify that you must battle with the same species, but it doesn’t say whether they have to be their native Kantonian form or their Alolan form instead. If you have the opportunity, consider using an Alolan form Pokémon. Of course, only do so if the Alolan form has a noticeable advantage; don’t use an Ice-type Alolan Vulpix against the Fire-type Vulpix…
Looking at the match-ups, good Alolan form Pokémon to use include Raichu (for Psychic STAB to get around the Electric vs Electric deadlock), Grimer/Muk (Dark STAB for a similar reason) and Exeggutor (Dragon STAB and takes even less damage from the opposing Exeggutor).
3. If All Else Fails, Rely on Hard Work
In situations where you can’t play a little dirty, there’s nothing for it but to put in some extra work. You might not need max IVs like your opponents, but it would help a lot if your Pokémon were as high Level as possible and if you feed your Pokémon stat-boosting candies to boost their AVs. The best way to achieve both is to create huge Catch Combos, ideally with the Pokémon of the same species as the one you’re training.
This way, you’ll earn a lot of experience points for your party, plus lots of stat-boosting candy. If the Pokémon you’re catching are the same species, you may also receive species candy, which boost all stats. Depending on the rarity of the Pokémon, this may be easier or harder for some. But you’ll still get general stat-boosting candy (eg. Courage Candy), which are useful enough.
4. Try to Plan Ahead with Evolutions
This isn’t advice to win battles, but when tackling Master Trainers, if you’re going to train one Pokémon all the way, it’s much quicker if you use that same Pokémon for multiple Master Trainers (if possible). For instance, if you’re attempting the Venusaur Master Trainer, you may as well train a Bulbasaur, so you can defeat the Bulbasaur trainer. Then evolve the Bulbasaur into Ivysaur to defeat that trainer. Before challenging the Venusaur trainer.
With that said, make sure to give enough space Level-wise, so you can actually evolve your Pokémon after they’ve done their work! So in Bulbasaur’s case, don’t go above Level 99, as you won’t have enough Levels to make it evolve into Venusaur. This won’t be a problem for Pokémon that don’t evolve via Level Up though, like Pokémon that require evolution stones. Regardless, it doesn’t hurt to double check!