Community Day Decidueye is a very strong Grass-type attacker, and it will be especially useful against other Grass-type Pokémon. Hisuian Decidueye, on the other hand, is a more specialized attacker that is best used against Psychic and Ghost-type Pokémon.
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There is no one-size-fits-all answer to this question, as the best team will depend on your own Pokémon and their levels. However, some strong counters to Dartrix include Swampert, Roserade, and Mamoswine.
If you evolve Dartrix into Decidueye from 2 p.m. until 10 p.m. in your local time, it will learn the charged move Frenzy Plant.