Demon's Souls: Recommended early stats leveling path for Magician class
While I cannot say I have hands on experience with the Magician class (other than trying it out for a bit), several recommended builds suggest that what you really want to up, at least in the long run, is MAG.
Of course, this is not what you wanted to know, but I did find this quote from a discussion regarding the possibility of playing a pure magic user (some spoilers if you don't want to know advanced spell names):
Speaking as a pure mage with maxed out MAG and endgame magic EQ: it's completely possible.
You will encounter a lot of difficulties during your first playthrough. But you can't just endlessly pump points into MAG, sometimes you have to deviate. Dont hesitate putting points into HP or Endurance (for armor).
At around level 180~, you will see that the game basically play itself with Moving Soul Arrows, and Firestorm.
I started out as a noble (L1) and poured everything into the magic skill until it was about 27.
It was awesome for a bit because I could kill the metal skelatons in 2 hits in the storm place and rack up even more souls which i poured further into magic.
Then I ran into a snag. I was leaping off the cliffs in the storm place after killing the skeletons (soul mining), but caused the world tendancy to shift all the way to black. THis caused a red demon version of the Japanese dude to spawn at the very beginning of the storm place. He is basically immune to magic, proceeded to serve me my head on a stick, and put a serious damper on my soul mining.
The moral of the story is pour all your points into magic (nothing else) until about level 24. Then start putting some into vitality and strength (go to str 12) to round off your character a bit.