This post contains major spoilers for the boss Nyx the Forgotten One.
Nyx The Forgotten One is a very tricky boss as when she is battled the types that are effective or not effective against others are inverted.
Here’s a type chart that demonstrates this concept:
Given this, it is extremely unwise to use Pokémon with many resistances as all those resistances will turn into weaknesses. Also, Pokémon with many weaknesses will have many resistances and are ideal for this fight. A few great examples of Pokémon like this are: Delta Gardevoir, Delta Scizor, and Gallade. All 3 of these Pokémon can mega evolve and are excellent choices for this battle.
With Aurorus, normal, ice, poison, and flying type moves are super effective against it. Water, grass, fighting, ground, rock, and steel type moves are not very effective against it.
With Breloom, water, electric, grass, ground, rock, and dark type moves are super effective against it. Fire, ice, poison, flying, psychic, and fairy type moves are not very effective against it.
With Greninja, fire, water, ice, psychic, ghost, dark, and steel type moves are very effective against it. Electric, grass, fighting, bug, and fairy type moves are not very effective against it.
With water Greninja, fire, water, ice, and steel type moves are super effective against it. Electric and grass type moves are not very effective against it.
With ice Greninja, ice type moves are super effective against it. Fire, fighting, rock, and steel type moves are not very effective against it.
With dark Greninja, psychic, ghost, and dark type moves are super effective against it. Fighting, bug, and fairy type moves are not very effective against it.
With grass Greninja, water, electric, grass, and ground type moves are super effective against it. Fire, ice, poison, flying, and bug type moves are not very effective against it.
With Garchomp, fire, electric, poison, and rock type moves are super effective against it. Ice, dragon, and fairy are not very effective against it.
With Exeggutor, water, electric, grass, fighting, ground, and psychic type moves are super effective against it. Fire, ice, poison, flying, bug, ghost, and dark type moves are not very effective against it.
With Kangaskhan, ghost types are super effective against it. Fighting type moves are not very effective against it.
If you’re having trouble fighting Nyx it is advisable to teach TM’s to your Pokémon so that they contain as many super effective moves as possible. Also, using choice items such as choice specs, choice bands, and choice scarfs will make the fight significantly easier because, as long as your Pokémon outspeed her Pokémon and have a super effective move boosted by the choice item, you can OHKO many of her Pokémon before they become a threat.