Not having played a mage, I think Firewall would be the best non-arrow-like spell to use, which gives a high damage, short range area effect around the caster (short compared to an arrow, anyway; I tried it once and IIRC the range was 2 or 3 times what you could reach with a one-handed sword).
The
walkthrough contains the description and details of all the skills / spells, with comments on their effectiveness. I didn't read through them all, but with the ones I did check the comments seem generally reasonable.
