Good thread.
Contrary to what most people are saying here, I found soloing a mage incredibly easy. It all depends on how you play it. My main spells were:
Blind
Mirrored Image
Dire Charm
Any blinded enemy is a dead enemy, as they just stand there and you can pelt them with bullets. I usually dire charmed tough fighters and used them to kill their friends, then when they were done, I blinde them and killed them... muahaha
Mirrored image is an awesome spell. You're basically immune for a few rounds while they hit your ghost images. By that time they're either charmed or blinded.
Of course it's nice to toss a few fireballs when there's a group of enemies tightly grouped, but I only used it a few times.
Your mileage may vary.