Every game that separates elements runs into this trap - before you even kick a ball you've got 4 spellcaster trees to worry about.
They should've combined Fire and Earth, and Aero/Hydro and added a 2nd Str tree (taken some man at arm spells and put them into a Paladin tree or whatnot) to achieve a better balance.
Then there would be:
Man at Arms/Paladin for Str
Marksman/Scoundrel for Dex
Magma/Storm/Witchcraft for Int (Magma being fire/earth, Storm being hydro/aero)
Had they done this, you'd be in a better place to balance the initial spell offerings and subsequent spell fields, so each had a similar number per level - creating better balance between attributes.
