Skylanders Giants

This achievement requires you to open any of the Elemental areas that you come across in the game. If you don't know what these areas are, there will be a symbol on the ground showing you what element will unlock the area. Each level has 3 different elemental areas so you should have no problem coming across one eventually throughout your campaign, If you have any further questions feel free to leave a comment.

This achieveable within the first level.
This refers to the area where you will require a Skylander of a certain element to open.
You can use either one of your new Skylanders or any of your old ones from Series 1.
In the first level before exiting the cave you will approach an area with a grass element symbol on the ground and when you walk over to it (without a grass element) Skylander it will place some vines in your way.
But we are in luck, the Giant you just got (Tree Rex) with your new portal is a grass element so switch to him on the Portal and the area will open and this will pop.

Note:
There are 2 other areas in this level (One of Air and one of Undead). If you also unlock these you will get the following achievement:

The Elemental Enthusiast achievement in Skylanders Giants worth 15 pointsChapter 1 - Open all the Elemental areas