I actually think he is pretty balanced.
Let's look at his pros. He has an attack that does 7 dice of damage, an area attack that does 3 dice of damage, tons of life, he is burnproof, he has lightning -2, he has tough -2, and he is unmovable.
Let's look at his cons. He costs 20 mana, which is pretty steep for the Warlord. He isn't a soldier, which means a lot of the Warlord's abilities don't apply to him. He is slow. He doesn't have any quick actions, which limits his attack options and doesn't give him the counterstrike ability when he guards. His area attack has absolutely no range and can harm his own troops. He has no armor and can't gain any armor. He is nonliving, so can't be the target of spells that require the target to be living. Finally, no psychic immunity.
>tl;dr
Earth Elemental doesn't seem OP to me at all, especially with all the ways to deal with powerful creatures. Just quicksand him, tanglevine him, banish him, or keep teleporting him away. It isn't like he is unstoppable. I actually find Iron Golem to be more playable in my games. Earth Elemental rarely lasts long because he becomes such a huge target.
