Earth.

Earth is defense, strength, but also Wisdom. IT's been around for a while. It's's seen the other elements churn and change while it is slowly changes with the help of the others. While it can attack, it can build it's stregh over time, to unlesh fury, or even simply outlast an opponent.

Earth Cracs. Stone Shatters. Only the soul is truly eternal - Randroth Elder Earth Dragon