Delta Gardevoir is rly helpful against nyx, I suggest USing that with the following set
Delta Gardevoir @LifeOrb
252 SpAtk/252 Speed
Timid Nature
T-Bolt
Ice Beam
Earth Power/Moon Blast/ Shadow Ball
Calm Mind
I beat nyx by just using this thing tbh
apparently, “Grass/Psychic, Grass/Ice, and Grass/Dark have the most weaknesses” so use one of these btw i havent played the game much but i know types are reversed in this battle.