A guide on how to take on the second Totem Pokémon in the game: Totem Wishiwashi!
Totem Wishiwashi is a level 20 Water-type Pokémon, and has the following moves:
Water Gun is a Water-type move that deals damage, while Growl lowers the target’s Attack by one stage, making their physical attacks weaker. Rain Dance is a move that sets the rain weather condition up; it causes several effects, with the most notable being increasing damage done by Water-type attacks and decreasing damage done by Fire-type attacks by 50%. Soak changes the target’s type to Water.
Thanks to its Schooling Ability, Wishiwashi starts the battle in its powerful School Form. In this form its Attack, Defense, Special Attack, and Special Defense is a lot higher than when it's in its Solo Form. It reverts to its Solo Form when its HP is below 25%, lowering the named stats.
Totem Wishiwashi's aura gives it a boost of +1 Defense, and it holds a Sitrus Berry, which heals it for 25% of its HP when it's below 50%.
Totem Wishiwashi can call two allies. One of them is a level 18 Solo Form Wishiwashi, and has the following moves:
Helping Hand increases the damage done by the ally Wishiwashi's attack by 50% the turn it is used. Feint Attack is a Dark-type move that never misses. Brine is a Water-type attack that deals increased damage if the target’s HP is below 50%, and Aqua Ring will heal the target for a small amount of their HP for a few turns after being used.
This Wishiwashi also has the Schooling Ability, but stays in its Solo Form as the Ability only activates when the Pokémon is level 20 or higher.
The other ally that can be called is a level 18 Alomomola, a Water-type Pokémon that has the the following moves:
Helping Hand and Water Gun are explained above. Double Slap is a Normal-type move that may hit 2-5 times, and Heal Pulse heals every Pokémon on the field.
Alomomola's Healer Ability gives the Pokémon a 30% chance to heal a status condition on its ally (Totem Wishiwashi) at the end of each turn.
Grass is the best option as it can hit for super-effective damage as well as resist Water attacks.
Electric can hit for super-effective damage but doesn’t resist, while Water and Dragon resist but do not hit for super-effective damage (although this may not be very important as your Pokémon may be hit by Soak in which case the same will apply).
The best Pokémon for this battle is a Finneon with the Ability Storm Drain, which makes it draw in all Water-type moves, making it impossible for Totem Wishiwashi to deal damage to it. You can catch one while surfing in Brooklet Hill, outside the Totem’s Den. You may want it to be at least level 17 so it gets Gust, or teach it some TMs.
Steenee is a good Grass-type option. Its pre-evolution Bounsweet can be obtained just a bit before this trial, by trading it for a Lillipup with the Rising Star in the Route 5 Pokémon Center. Bounsweet evolves into Steenee at level 18; this means it will have the stats of a freshly-evolved Pokémon.
The starter Pokémon Rowlet is also a good Grass-type option, and evolves into Dartrix at level 17. Note that it is advisable to not overwrite the move Leafage until after this battle (keep both Razor Leaf and Leafage for a while).
Drifloon is a good option if you have been using it for the previous boss battles as well, as it gets access to the Electric-type move Charge Beam, available as a TM just before Brooklet Hill; Charge Beam incrases the user's Special Attack by one stage each time it is used, meaning each successful use of it is stronger.
Pikachu is a good Electric-type that can also make use of Charge Beam.
As with almost all Totem Pokémon battles, it is advisable to focus on the Totem first and the ally later. In this case, if you are lucky enough to get Solo Form Wishiwashi called first, it is quite important that you do not make it faint so that Alomomola never appears. For Dartrix (and other Grass-type) users, make sure not to use Razor Leaf until the Totem Pokémon is in KO range, as it will KO Solo Form Wishiwashi.
The reason for this is that Alomomola has access to Heal Pulse which can heal Totem Wishiwashi. This is more of a problem if you damage it enough to force it into its Solo Form, as said heal may give it enough HP to revert to School Form again. It also has the Healer Ability which makes it harder to keep a status condition on Wishiwshi.
The Solo Form Wishiwashi ally, on the other hand, is not a major threat as its stats are extremely low; even Brine on a damaged Pokemon shouldn’t hit as hard as Totem Wishiwashi’s Water Gun. The only help it can really bring is Helping Hand, but that’s something Alomomola can do too.
Totem Wishiwashi itself is very challenging (with an exception), though. With its good stats, under rain, and a Helping Hand, Water Gun hits a lot. Soak makes the target resistant to Water Gun (even though it can still hit hard enough) but it also rids them of their STAB if they are not Water-type. Finneon with the Storm Drain Ability, however, is immune to Water-type attacks entirely, which makes Wishiwashi unable to damage it. It can then damage it little by little.
If you do not use Storm Drain Finneon, you will probably have to either overlevel your Pokemon (which can be done somewhat easily with the help of SOS battles) or use Grass-types. Reviving them over and over through the use of dummy Pokémon can also work.