The easiest way would be:
Level 1 Necromancery (black)
Level 1 Force
Wisdom +1
Level 2 Force
You might try to be an elementalist but do not take Water first.
Nature magic is good too but forget about clervoyance. Summoning is fun too.
If you want to go solo (no party), you'll need Heal. Soloing mage is possible.
There are several mages in the game and Virgil can grow strong.
Now a cryptic hint:
The best weapon for a mage is a bladed weapon you can buy in a place of cold mountains and home of a mythical creature.
