Cleric-like character are getting pretty powerful, once they hit a decent STR/INT (+PER/SPD) value...
Re-rolled though, since earthmagic is kinda 'boring' - gave my new cleric witchcraft + 1 point fire (haste!!)
Earth on the other hand has all the ultimate spells right at lvl 1: spider (!), Fortify & oil.... - that is, what Jahan is going to learn in my new campaign ^^
You dont need air, cause of Jahan - witchcraft really is a bit underestimated, since it can hugely buff you or your parties' damage or debuff the boss'. Since am not that far into with my current party, I cant say anything about the summons, but read they are really powerful as well...
(for a cleric, your best bet is to take the one talent that boosts your int right away)
Uhm I played up to lvl 11 with my cleric, and had Earth magic - as you might have guessed already too. The only spell that really is cool later on is the rock-ball-thingie spell, else poison is mostly useless or hinders yourself...your summons are situational effective - poison is not. For the oil crap to be ultimately effective, you'd need some cheap fire magic or some legendary ring.
edit: else; play for a bit and learn at your own mistakes

and do whatever to your liking...
As said above, dual-role-characters can be very powerful (and will be)