"Elemental" is not a subtype. The Elemental is its own type of creature, it is completely different from an outsider.

It also makes no sense that you give the Forestwalker form and the Elemental Fury form the same damage with their natural slam attack, when the Elemental Fury form is an entire Size category larger than the creature. Yes, you add a d6 of energy damage, but you make the same natural attack deal the same damage for two different sized creatures. It just...doesn't make sense.

The picture is awesome.

The bonus to ability scores should be "inherent", not "inherit".

Natural Spell should have a "Divine Caster Level 5" prerequisite, and should only apply to divine spells, to prevent theurge and single-level dip shenanigans.

Natural Casting, as written, allows you to cast spells like control weather, restoration, liveoak, and even awaken as a single standard action. Was this intentional?

The class, overall, looks decent. I would still remove the animal companion, personally, but that's just me.