Goblin King is in the ancient forest. When you encounter the scaffolding area with Goblins, go up up up and then go to the right. He will have two goblin minions with him. I used Yowie, Imori, and Catzerker. I used buffs on him: chill from Yowie, poison from Imori, and bleed from Catzerker. It was an easy fight for my level 25 squad.