Nathan went to Route 3 so he can train a bit. Chimchar first began fighting wild Pokémon until he got strong enough to fight trainers. They eventually cleared Route 3, but Nathan thought that he would've needed to train more since he only had one Pokémon. He used some of his Potions to heal Chimchar and started heading to the forest. "C'mon, we gotta win this badge, right?" Nathan asked Chimchar with a smile. The little monkey responded with a determined nod. When they got to Santalune Forest, Chimchar got scared at the darkness. "Hey, it's okay. I'll be here," Nathan told him. And with that they began fighting the trainers there as well.
The entire process took about 2 hours, but Chimchar and Nathan were happy with the results. In addition to getting stronger, Chimchar learned Ember, Taunt, and Fury Swipes. Nathan headed back to the Pokémon Center with Chimchar, so they could be completely ready to battle the gym leader.