After creating the blue flame in the cave in Act 2, climb the vines to get to the area by the stone door. Leave the cave and climb down the vines as the Wakcha walks over to the right. Use the wooden cover on the cooking pot to create a distraction, then sneak towards the vase and get in it. Because Teku didn't extinguish his flame, there will be a blue glow coming out of the vase. The Wakcha will notice this and will capture Teku, but you'll get the achievement.