You seem to be familiar with the Arc area, so it's not a spoiler to say have high shock resist, and be able to do high shock damage. Arc gear can be quite helpful in that regards.
Mage Armour / Mage Barrier can be quite helpful.
Depending on your build and playing style, maxed out Disintegrate can prove devastating.
Even if you're not a true Summoning build, Lessor Summoning proves useful (Even a Level 1 sparkling is helpful). If you are going the summoning route, maxed out Earth Mastery and Summoner is impressive. An Level 18 Iron golem is almost unfair in the arc.
So for exact specifics:
Max out the following (or as close to it as possible)
Mage Skills: Lightening Bold, Mage Armour, Staff Mastery, Lessor Summoning
Advanced Skills: Disintegrate, Mage Barrier, Earth Mastery, Summoner.
Drop extra Skill Points (if any) into: Precision Shots. Mana Surge, Anarchist.
Obviously invest heavily in INT and PERS for mana. But don't be afraid to invest into AGL and END if you'd like a slightly more durable mage. I've seen some Mages with INT, AGL and END having equal value (5, or 6). Obviously, how many trait points you have is totally level dependent.
Arc equipment and anything that gives sizable shock protection or trait bonus you really like. Some mages like using fire protection boots just as a way to soften up the damage if they get hit by an explosion.
And for clarity, Good, or Evil ending isn't much a factor in how you build your mage as the traits tend to be high enough. But, going for the combat ending rather than the peaceful ending would require you to be able to fight a certain end game entity. That might impact your build.