You have to capture coballion first. Coballion is located at the end of mistralon cave. Mistralon cave is on route 6, right before chargestone cave. When you get to a house, there are two paths. They both lead to chargestone cave, but take the path on the right. You will get to a river, and you must have surf. Use surf to go down the river. When you reach two rocks blocking the river, there is a short strip of land touching the river. Ust the strip of land to go around the rocks. Keep going, and you will reach Mistralon cave. Coballion lies at the end of the cave. Once you capture coballion, you might be able to catch terrakion. If you can't, then you need to capture virizion, which does requires catching coballion. You can find virizion in pinwheel forest. Follow the main road, but watch the right side of it. When you see an opening in the fence, go through the opening. Follow the path, and you will reach an entrance to ruminition field. Virizion lies there. After catching virizion, you will be able to catch terrakion.
