It would be an interesting challenge to try it as a pure mage, since in my playthroughs, I mainly concentrated on Aardt and Igni, and therefore did not develop the other signs to their full potential. The only problem might be that you might find yourself running short of Endurance (mana) at times, even with potions...
I did have a lot of fun killing monsters from a distance with Igni only in the swamps, but I'm not so sure about big mobs and bosses...
When you say "pure" mage, do mean not using weapons at all, or simply not developing them, in favour of rather developing the Signs?