hum
fire is good
lightning is if you want an extra nuking damage
ice is good if you want to slow down the enemy and hit him hard(chance of freezing=cancels skills/magics on the spot)
so fire+ice=low, but effective
and fire+lightning=strong, bit less effective then fire+ice
And about nature, its your decision

You can take what ever you want(ofcourse) but you must know, Nature is pretty weak :S
No offensive attacks, if you are the party type guy, go on Nature,if you arnt that type of guy(like i am) dont go on that!