Elemental Enthusiast Achievement
Chapter 1 - Open all the Elemental areas
There are three elemental areas to find, and you will need a Life Skylander, an Air Skylander, and an Undead Skylander to unlock them all. (These are the three types included in the starter pack.)
The life elemental area is found just after the Chompy pit, before you leave the cave. The air elemental area is to the left, just after exiting the cave. The undead elemental area is accessible after you break the first chain on the robot. Circle left around the small building below you, and you'll find the area.
Here's a video by XboxAchievables: