You face one of these three depending on your starter. Cilan (Grass) is weak to Fire, Chili (Fire) to Water, and Cress (Water) to Grass. Their ace is a Pansear, Pansage, or Panpour. Use the elemental monkey given to you in the Dreamyard that is super effective against your chosen Gym Leader. Their Pokémon are low level, so any strong super-effective attack should suffice.
