Earth Elementals are weak to fire spells as some have pointed out. To boost your spell damage you want to maximize your Evaluate Intelligence and GM Inscription for more damage which was also pointed out. To get quicker damage increases with equipment use an earth elemental slayer spellbook and put on some items with Spell Damage Increase.
For faster spellcasting of course maximize your Fast casting 2 and Fast Cast Recovery 6 if you are straight mage. Then of course everything else is about more mana, faster mana and did I mention more mana. Maximize all these whereever and whenever you can: Lower Mana Cost, Maximize Mana Regeneration, Mana Increase, and Intelligence Increase. And of course there is always Lower Reagent Cost so you don't have to carry around reagents. Once you do all this and build a set of slayer spellbooks for all the different creatures in game you should be able to reduce most creatures to ash heaps very quickly. Spend some time in the Hunters Guide learning what creatures are weak to what spells, that knowledge will be very helpful in the long run. Enjoy