In battle, Chewies attack physically by biting. They are also capable of using moves like Drain and can use a nasty move called Pollen Nap that may put one of Mario's party to sleep.
Chewies have no true weakness but they tend to appear in groups. It is recommended that Mallow uses a multi-hit move like Thunderbolt to damage all of them at the same. Mario should focus using physical and Geno could also multi-hit using his Geno Blast or Geno Flash moves.